release(v4.6.1): bind globalThis.fetch in browser-receiver-sensitive call sites

Browsers' Window.fetch is a WebIDL bound operation; storing it as
this.fetchImpl / this.fetchFn and calling via the instance receiver
threw "Illegal invocation" on the first request. Bind once at
construction in InboxClient, LongPollBridge, and SseBridge. Reported
by Prism (multi-device E2EE terminal), blocking every browser
consumer of the v4.6 transport stack on inbox.start() / bridge.connect().

WsBridge unaffected (uses WebSocket). Node/Bun fetch tolerates a free
receiver, so the bug never surfaced server-side — added regression
tests that install a strict-receiver globalThis.fetch to catch the
issue without an actual browser harness.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/shade-transport-bridge/src/sse-bridge.ts

@@ -58,7 +58,10 @@ export class SseBridge implements BridgeTransport {
     console.warn('[shade-bridge:sse]', err.message);
 
   constructor(private readonly options: SseBridgeOptions) {
-    this.fetchFn = options.fetch ?? globalThis.fetch;
+    // Bind to globalThis: browser `fetch` is a WebIDL bound operation
+    // and throws "Illegal invocation" when called as `this.fetchFn(...)`.
+    const f = options.fetch ?? globalThis.fetch;
+    this.fetchFn = f.bind(globalThis);
     this.cursor = options.startCursor ?? 0;
   }