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

1 files changed, 92 insertions, 8 deletions

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 });
+          }
+        };
+      }
+    };
   };
 }