Implement v0.4 subscriptions + v0.5 theming and plugin slots

- SSE and WebSocket fetchers via graphql-sse and graphql-ws,
  returning AsyncIterable results
- SessionStore.run consumes AsyncIterable streams with
  subscriptionMode "append" (timestamped) or "replace" and
  honors an AbortSignal for cancellation
- Chrome CSS variables documented in styles/theme.scss with
  prefers-color-scheme light/dark gating and .graphiql-light override
- Svelte 5 snippet slots on GraphiQL: toolbarExtras, tabExtras,
  resultFooter

9 files changed, 334 insertions, 28 deletions

diff --git a/source/library/GraphiQL.svelte b/source/library/GraphiQL.svelte
index 0a8f01a..329dd6f 100644
--- a/source/library/GraphiQL.svelte
+++ b/source/library/GraphiQL.svelte
@@ -1,7 +1,7 @@
 <script lang="ts">
   /*** IMPORT ------------------------------------------- ***/
 
-  import { onMount } from "svelte";
+  import { onMount, type Snippet } from "svelte";
 
   /*** UTILITY ------------------------------------------ ***/
 
@@ -17,6 +17,7 @@
   import { HistoryStore } from "./state/history.svelte.ts";
   import { SchemaStore } from "./state/schema.svelte.ts";
   import { SessionStore } from "./state/session.svelte.ts";
+  import type { SubscriptionMode, Tab } from "./state/session.svelte.ts";
   import { createLocalStorage, createMemoryStorage } from "./state/storage.ts";
   import type { Storage } from "./state/storage.ts";
 
@@ -24,16 +25,24 @@
     fetcher: Fetcher;
     initialQuery?: string;
     namespace?: string;
+    resultFooter?: Snippet<[{ result: string }]>;
     storage?: Storage;
+    subscriptionMode?: SubscriptionMode;
+    tabExtras?: Snippet<[{ tab: Tab }]>;
     theme?: Extension;
+    toolbarExtras?: Snippet;
   };
 
   let {
     fetcher,
     initialQuery = "",
     namespace = "eol-graphiql",
+    resultFooter,
     storage,
-    theme
+    subscriptionMode = "append",
+    tabExtras,
+    theme,
+    toolbarExtras
   }: Props = $props();
 
   const resolvedStorage = storage ??
@@ -87,15 +96,23 @@
     schema.introspect(fetcher);
   });
 
+  let runAbort: AbortController | null = null;
+
   async function run() {
-    if (running)
+    if (running) {
+      runAbort?.abort();
       return;
+    }
 
     running = true;
+    runAbort = new AbortController();
 
     try {
       const tab = session.active;
-      const ok = await session.run(fetcher);
+      const ok = await session.run(fetcher, {
+        signal: runAbort.signal,
+        subscriptionMode
+      });
 
       if (ok && tab) {
         history.add({
@@ -107,6 +124,7 @@
         });
       }
     } finally {
+      runAbort = null;
       running = false;
     }
   }
@@ -247,6 +265,7 @@
     disabled={running || !session.active}
     docsAvailable={schema.schema !== null}
     {docsOpen}
+    extras={toolbarExtras}
     {historyOpen}
     onRun={run}
     onSelectOperation={(name) => {
@@ -261,6 +280,7 @@
     schemaLoading={schema.loading}/>
   <TabBar
     activeId={session.activeId}
+    extras={tabExtras}
     onAdd={() => session.addTab()}
     onClose={(id) => session.closeTab(id)}
     onRename={(id, title) => session.renameTab(id, title)}
@@ -310,7 +330,7 @@
       </div>
     </div>
     <div class="right">
-      <ResultViewer {theme} value={session.active?.result ?? ""}/>
+      <ResultViewer footer={resultFooter} {theme} value={session.active?.result ?? ""}/>
     </div>
     {#if docsOpen && schema.schema}
       <DocExplorer schema={schema.schema}/>
diff --git a/source/library/components/ResultViewer.svelte b/source/library/components/ResultViewer.svelte
index e2c74fe..083828d 100644
--- a/source/library/components/ResultViewer.svelte
+++ b/source/library/components/ResultViewer.svelte
@@ -1,13 +1,15 @@
 <script lang="ts">
   import Editor from "./Editor.svelte";
   import type { Extension } from "@codemirror/state";
+  import type { Snippet } from "svelte";
 
   type Props = {
+    footer?: Snippet<[{ result: string }]>;
     theme?: Extension;
     value: string;
   };
 
-  let { theme, value }: Props = $props();
+  let { footer, theme, value }: Props = $props();
 
   function noop(_v: string) {}
 </script>
@@ -15,7 +17,7 @@
 <style lang="scss">
   .result {
     display: grid;
-    grid-template-rows: auto 1fr;
+    grid-template-rows: auto 1fr auto;
     height: 100%;
     min-height: 0;
   }
@@ -27,9 +29,19 @@
     padding: 0.25rem 0.75rem;
     text-transform: uppercase;
   }
+
+  .footer {
+    background: var(--graphiql-panel, #252526);
+    border-top: 1px solid var(--graphiql-border, #333);
+    font-size: 0.75rem;
+    padding: 0.25rem 0.75rem;
+  }
 </style>
 
 <div class="result">
   <div class="label">Response</div>
   <Editor language="json" onChange={noop} readOnly {theme} {value}/>
+  {#if footer}
+    <div class="footer">{@render footer({ result: value })}</div>
+  {/if}
 </div>
diff --git a/source/library/components/TabBar.svelte b/source/library/components/TabBar.svelte
index d87449d..9c34f20 100644
--- a/source/library/components/TabBar.svelte
+++ b/source/library/components/TabBar.svelte
@@ -1,9 +1,10 @@
 <script lang="ts">
-  import { tick } from "svelte";
+  import { tick, type Snippet } from "svelte";
   import type { Tab } from "../state/session.svelte.ts";
 
   type Props = {
     activeId: string;
+    extras?: Snippet<[{ tab: Tab }]>;
     onAdd: () => void;
     onClose: (id: string) => void;
     onRename: (id: string, title: string) => void;
@@ -11,7 +12,7 @@
     tabs: Tab[];
   };
 
-  let { activeId, onAdd, onClose, onRename, onSelect, tabs }: Props = $props();
+  let { activeId, extras, onAdd, onClose, onRename, onSelect, tabs }: Props = $props();
 
   let editingId = $state<string | null>(null);
   let draft = $state<string>("");
@@ -150,6 +151,7 @@
       {:else}
         <span class="title">{tab.title}</span>
       {/if}
+      {#if extras}{@render extras({ tab })}{/if}
       <button
         aria-label="Close tab"
         class="close"
diff --git a/source/library/components/Toolbar.svelte b/source/library/components/Toolbar.svelte
index a17191c..8c75668 100644
--- a/source/library/components/Toolbar.svelte
+++ b/source/library/components/Toolbar.svelte
@@ -1,10 +1,12 @@
 <script lang="ts">
   import type { OperationInfo } from "../graphql/operations.ts";
+  import type { Snippet } from "svelte";
 
   type Props = {
     disabled: boolean;
     docsAvailable?: boolean;
     docsOpen?: boolean;
+    extras?: Snippet;
     historyOpen?: boolean;
     onRun: () => void;
     onSelectOperation?: (name: string | null) => void;
@@ -20,6 +22,7 @@
     disabled,
     docsAvailable = false,
     docsOpen = false,
+    extras,
     historyOpen = false,
     onRun,
     onSelectOperation,
@@ -125,6 +128,7 @@
       {/each}
     </select>
   {/if}
+  {#if extras}{@render extras()}{/if}
   <span class="hint">⌘/Ctrl + Enter</span>
   {#if schemaLoading}
     <span class="hint">Loading schema…</span>
diff --git a/source/library/fetcher/sse.ts b/source/library/fetcher/sse.ts
index b6805dc..c28436b 100644
--- a/source/library/fetcher/sse.ts
+++ b/source/library/fetcher/sse.ts
@@ -1,18 +1,102 @@
 
 
 
+/*** IMPORT ------------------------------------------- ***/
+
+import { createClient, type Client, type ClientOptions } from "graphql-sse";
+
 /*** UTILITY ------------------------------------------ ***/
 
-import type { Fetcher, FetcherOptions } from "./types.ts";
+import type { Fetcher, FetcherOptions, FetcherResult } from "./types.ts";
 
 /*** EXPORT ------------------------------------------- ***/
 
-/**
- * Server-Sent Events fetcher for graphql-sse protocol.
- * Stub implementation — see PLAN.md stage v0.4 for full implementation.
- */
-export function createSseFetcher(_options: FetcherOptions): Fetcher {
-  return () => {
-    throw new Error("SSE fetcher not yet implemented — see PLAN.md v0.4");
+export type SseFetcherOptions = FetcherOptions & {
+  client?: Partial<Omit<ClientOptions, "url">>;
+};
+
+export function createSseFetcher(options: SseFetcherOptions): Fetcher {
+  const client: Client = createClient({
+    fetchFn: options.fetch,
+    headers: options.headers,
+    url: options.url,
+    ...options.client
+  });
+
+  return (req) => {
+    return {
+      [Symbol.asyncIterator](): AsyncIterator<FetcherResult> {
+        const queue: FetcherResult[] = [];
+        const resolvers: ((step: IteratorResult<FetcherResult>) => void)[] = [];
+        let done = false;
+        let error: unknown = null;
+
+        const dispose = client.subscribe<FetcherResult, Record<string, unknown>>(
+          {
+            extensions: req.headers as Record<string, unknown> | undefined,
+            operationName: req.operationName ?? undefined,
+            query: req.query,
+            variables: req.variables
+          },
+          {
+            complete() {
+              done = true;
+              flush();
+            },
+            error(err) {
+              error = err;
+              done = true;
+              flush();
+            },
+            next(value) {
+              queue.push(value as FetcherResult);
+              flush();
+            }
+          }
+        );
+
+        function flush() {
+          while (resolvers.length > 0 && (queue.length > 0 || done)) {
+            const resolve = resolvers.shift();
+
+            if (!resolve)
+              break;
+
+            if (queue.length > 0)
+              resolve({ done: false, value: queue.shift() as FetcherResult });
+            else if (error)
+              resolve({ done: true, value: undefined });
+            else
+              resolve({ done: true, value: undefined });
+          }
+        }
+
+        return {
+          next(): Promise<IteratorResult<FetcherResult>> {
+            if (error) {
+              const err = error;
+              error = null;
+              return Promise.reject(err);
+            }
+
+            if (queue.length > 0)
+              return Promise.resolve({ done: false, value: queue.shift() as FetcherResult });
+
+            if (done)
+              return Promise.resolve({ done: true, value: undefined });
+
+            return new Promise<IteratorResult<FetcherResult>>((resolve) => {
+              resolvers.push(resolve);
+            });
+          },
+          return(): Promise<IteratorResult<FetcherResult>> {
+            dispose();
+            done = true;
+            flush();
+            return Promise.resolve({ done: true, value: undefined });
+          }
+        };
+      }
+    };
   };
 }
diff --git a/source/library/fetcher/websocket.ts b/source/library/fetcher/websocket.ts
index 6376e76..6c6bfe8 100644
--- a/source/library/fetcher/websocket.ts
+++ b/source/library/fetcher/websocket.ts
@@ -1,18 +1,99 @@
 
 
 
+/*** IMPORT ------------------------------------------- ***/
+
+import { createClient, type Client, type ClientOptions } from "graphql-ws";
+
 /*** UTILITY ------------------------------------------ ***/
 
-import type { Fetcher, FetcherOptions } from "./types.ts";
+import type { Fetcher, FetcherOptions, FetcherResult } from "./types.ts";
 
 /*** EXPORT ------------------------------------------- ***/
 
-/**
- * WebSocket fetcher for graphql-ws protocol.
- * Stub implementation — see PLAN.md stage v0.4 for full implementation.
- */
-export function createWsFetcher(_options: FetcherOptions): Fetcher {
-  return () => {
-    throw new Error("WebSocket fetcher not yet implemented — see PLAN.md v0.4");
+export type WsFetcherOptions = Omit<FetcherOptions, "fetch"> & {
+  client?: Partial<Omit<ClientOptions, "url">>;
+};
+
+export function createWsFetcher(options: WsFetcherOptions): Fetcher {
+  const client: Client = createClient({
+    connectionParams: options.headers,
+    url: options.url,
+    ...options.client
+  });
+
+  return (req) => {
+    return {
+      [Symbol.asyncIterator](): AsyncIterator<FetcherResult> {
+        const queue: FetcherResult[] = [];
+        const resolvers: ((step: IteratorResult<FetcherResult>) => void)[] = [];
+        let done = false;
+        let error: unknown = null;
+
+        const dispose = client.subscribe<FetcherResult, Record<string, unknown>>(
+          {
+            extensions: req.headers as Record<string, unknown> | undefined,
+            operationName: req.operationName ?? undefined,
+            query: req.query,
+            variables: req.variables
+          },
+          {
+            complete() {
+              done = true;
+              flush();
+            },
+            error(err) {
+              error = err;
+              done = true;
+              flush();
+            },
+            next(value) {
+              queue.push(value as FetcherResult);
+              flush();
+            }
+          }
+        );
+
+        function flush() {
+          while (resolvers.length > 0 && (queue.length > 0 || done)) {
+            const resolve = resolvers.shift();
+
+            if (!resolve)
+              break;
+
+            if (queue.length > 0)
+              resolve({ done: false, value: queue.shift() as FetcherResult });
+            else
+              resolve({ done: true, value: undefined });
+          }
+        }
+
+        return {
+          next(): Promise<IteratorResult<FetcherResult>> {
+            if (error) {
+              const err = error;
+              error = null;
+              return Promise.reject(err);
+            }
+
+            if (queue.length > 0)
+              return Promise.resolve({ done: false, value: queue.shift() as FetcherResult });
+
+            if (done)
+              return Promise.resolve({ done: true, value: undefined });
+
+            return new Promise<IteratorResult<FetcherResult>>((resolve) => {
+              resolvers.push(resolve);
+            });
+          },
+          return(): Promise<IteratorResult<FetcherResult>> {
+            dispose();
+            done = true;
+            flush();
+            return Promise.resolve({ done: true, value: undefined });
+          }
+        };
+      }
+    };
   };
 }
diff --git a/source/library/index.ts b/source/library/index.ts
index 5aecff1..2bec6e4 100644
--- a/source/library/index.ts
+++ b/source/library/index.ts
@@ -24,5 +24,12 @@ export type {
 
 export type { HistoryEntry, HistoryInput } from "./state/history.svelte.ts";
 export type { OperationInfo } from "./graphql/operations.ts";
+export type { SseFetcherOptions } from "./fetcher/sse.ts";
 export type { Storage } from "./state/storage.ts";
-export type { Tab, TabSeed } from "./state/session.svelte.ts";
+export type {
+  RunOptions,
+  SubscriptionMode,
+  Tab,
+  TabSeed
+} from "./state/session.svelte.ts";
+export type { WsFetcherOptions } from "./fetcher/websocket.ts";
diff --git a/source/library/state/session.svelte.ts b/source/library/state/session.svelte.ts
index d9f52ff..9345d29 100644
--- a/source/library/state/session.svelte.ts
+++ b/source/library/state/session.svelte.ts
@@ -3,7 +3,7 @@
 
 /*** UTILITY ------------------------------------------ ***/
 
-import type { Fetcher } from "../fetcher/types.ts";
+import type { Fetcher, FetcherResult } from "../fetcher/types.ts";
 import {
   deriveTitle,
   parseOperations,
@@ -13,6 +13,12 @@ import type { Storage } from "./storage.ts";
 
 const STORAGE_KEY = "session";
 
+function isAsyncIterable<T>(value: unknown): value is AsyncIterable<T> {
+  return typeof value === "object" &&
+    value !== null &&
+    typeof (value as AsyncIterable<T>)[Symbol.asyncIterator] === "function";
+}
+
 type Snapshot = {
   activeId: string;
   tabs: Tab[];
@@ -40,6 +46,13 @@ export type TabSeed = {
   variables?: string;
 };
 
+export type SubscriptionMode = "append" | "replace";
+
+export type RunOptions = {
+  signal?: AbortSignal;
+  subscriptionMode?: SubscriptionMode;
+};
+
 export class SessionStore {
   activeId = $state<string>("");
   tabs = $state<Tab[]>([]);
@@ -115,12 +128,15 @@ export class SessionStore {
     tab.titleDirty = true;
   }
 
-  async run(fetcher: Fetcher): Promise<boolean> {
+  async run(fetcher: Fetcher, options: RunOptions = {}): Promise<boolean> {
     const tab = this.active;
 
     if (!tab)
       return false;
 
+    const mode = options.subscriptionMode ?? "append";
+    const signal = options.signal;
+
     try {
       const variables = tab.variables.trim() ? JSON.parse(tab.variables) : {};
       const headers = tab.headers.trim() ? JSON.parse(tab.headers) : {};
@@ -132,6 +148,39 @@ export class SessionStore {
         variables
       });
 
+      if (isAsyncIterable<FetcherResult>(result)) {
+        tab.result = "";
+        const iterator = result[Symbol.asyncIterator]();
+
+        try {
+          while (true) {
+            if (signal?.aborted) {
+              await iterator.return?.();
+              break;
+            }
+
+            const step = await iterator.next();
+
+            if (step.done)
+              break;
+
+            const payload = JSON.stringify(step.value, null, 2);
+
+            if (mode === "append") {
+              const stamp = new Date().toISOString();
+              const chunk = `// ${stamp}\n${payload}\n`;
+              tab.result = tab.result ? `${tab.result}\n${chunk}` : chunk;
+            } else {
+              tab.result = payload;
+            }
+          }
+        } finally {
+          await iterator.return?.();
+        }
+
+        return true;
+      }
+
       tab.result = JSON.stringify(result, null, 2);
       return true;
     } catch(err) {
diff --git a/source/library/styles/theme.scss b/source/library/styles/theme.scss
new file mode 100644
index 0000000..55f24aa
--- /dev/null
+++ b/source/library/styles/theme.scss
@@ -0,0 +1,47 @@
+// @eol/graphiql chrome theme variables.
+//
+// Override any variable under a scoped selector (e.g. `.graphiql`) or on :root
+// to re-skin the chrome. The editor theme is a separate prop (`theme?: Extension`).
+//
+// Supported variables:
+//   --graphiql-accent        primary action color (Run button, selected state)
+//   --graphiql-bg            base background (editor + result)
+//   --graphiql-border        divider lines between panes
+//   --graphiql-fg            primary foreground
+//   --graphiql-font          font-family for the chrome
+//   --graphiql-link          clickable links in docs / breadcrumbs
+//   --graphiql-muted         muted foreground (hints, timestamps)
+//   --graphiql-panel         panel background (toolbar, tabbar, headers)
+
+:root {
+  --graphiql-accent: #0e639c;
+  --graphiql-bg: #1e1e1e;
+  --graphiql-border: #333;
+  --graphiql-fg: #d4d4d4;
+  --graphiql-font: ui-monospace, SFMono-Regular, monospace;
+  --graphiql-link: #79b8ff;
+  --graphiql-muted: #858585;
+  --graphiql-panel: #252526;
+}
+
+@media (prefers-color-scheme: light) {
+  :root:not(.graphiql-dark) {
+    --graphiql-accent: #0366d6;
+    --graphiql-bg: #ffffff;
+    --graphiql-border: #e1e4e8;
+    --graphiql-fg: #24292e;
+    --graphiql-link: #0366d6;
+    --graphiql-muted: #6a737d;
+    --graphiql-panel: #f6f8fa;
+  }
+}
+
+:root.graphiql-light {
+  --graphiql-accent: #0366d6;
+  --graphiql-bg: #ffffff;
+  --graphiql-border: #e1e4e8;
+  --graphiql-fg: #24292e;
+  --graphiql-link: #0366d6;
+  --graphiql-muted: #6a737d;
+  --graphiql-panel: #f6f8fa;
+}