fix: the empty-new-session bug — three live client defects

Live-driven via the owner's dev server (browser tools), reproduced on
/chat/new: send fires, optimistic appends, POST never happens, and the
page shows the frozen empty state. Three compounding defects:

1. session-core: default fetch stored UNBOUND — the browser's native
   fetch requires this === Window, so every call threw 'Illegal
   invocation' and the POST /api/chat never fired. Node's undici has no
   receiver discipline, which is why 994/994 tests passed while the app
   was dead. Now defaulted through a receiver-preserving wrapper; pinned
   by a strict-this regression test (the browser's semantics simulated.

2. session.ts: state memo keyed on [model, forceRender] — the reducer
   dispatch is STABLE by contract, so the memo computed once per mount:
   no optimistic bubble, no stream events, no error banner. The view was
   frozen at first render. Rekeyed on a version counter bumped by the
   model's onChange.

3. view.ts: turn_end carries the clean 'Question posed' placeholder as
   a toolResult message (details: null) — applyResult flipped the quiz
   part to output-available with a null output and AnsweredCard crashed
   on answer.dismissed (killed the whole page render). Fork 2 now holds
   on the message-attachment path too: clean question results with null
   details are ignored; the AnsweredCard renders only the learner's real
   answer. Regression pin extended in session-core.test.ts.

Verified live end to end: new session → text send → probe questions in
the dock → ask answer → graded ✓ quiz card with explanation → reload
renders the full transcript from the native record with the pending
question intact.
EOF
)

components/session-core.ts

@@ -192,7 +192,11 @@ export class SessionModel {
 	constructor(opts: SessionModelOptions) {
 		this.chatId = opts.chatId;
 		this.base = opts.initialMessages;
-		this.fetch = opts.fetch ?? globalThis.fetch;
+		// BOUND: the browser's native fetch is a Window method — storing it
+		// unbound makes every call throw "Illegal invocation" in the browser
+		// (Node's undici doesn't care, so the tests never saw it). Always go
+		// through a receiver-preserving wrapper.
+		this.fetch = opts.fetch ?? ((...args) => globalThis.fetch(...args));
 		this.pollMs = opts.pollMs ?? TURN_POLL_MS;
 		this.waitMaxMs = opts.waitMaxMs ?? TURN_WAIT_MAX_MS;
 		this.onChange = opts.onChange;

components/session.ts

@@ -34,21 +34,27 @@ export function useChatSession({ chatId, initialMessages, initiallyInFlight }: U
 		modelRef.current = new SessionModel({ chatId, initialMessages });
 	}
 	const model = modelRef.current;
-	const [, forceRender] = useReducer((n: number) => n + 1, 0);
+	// The render VERSION — bumped by every model change, so the state memo
+	// below recomputes exactly when the model actually changed. (Using the
+	// reducer's dispatch as the memo dep was a freeze bug: dispatch is
+	// STABLE by contract, so the memo computed once per mount and the view
+	// never saw a single model update — no optimistic bubble, no stream, no
+	// error banner. The empty-session bug, 2026-09-26.)
+	const [version, bump] = useReducer((n: number) => n + 1, 0);
 	useEffect(() => {
-		model.onChange = forceRender;
+		model.onChange = bump;
 		return () => {
 			model.onChange = undefined;
 		};
-	}, [model, forceRender]);
+	}, [model, bump]);
 
 	// Reattach on mount (StrictMode-safe: a newer resume cancels the older).
 	useEffect(() => {
 		if (initiallyInFlight) void model.resumeStream();
 	}, [initiallyInFlight, model]);
 
-	// The view + state, derived per render from the model.
-	const state = useMemo<SessionState>(() => model.getState(), [model, forceRender] as const);
+	// The view + state, derived per model change from the model.
+	const state = useMemo<SessionState>(() => model.getState(), [model, version] as const);
 
 	// The reattach settled terminal → the sync-and-continue seam (chat.tsx).
 	const setReattachSettled = useCallback(