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;