fix: parallel question calls are asked, saved, and answered in the model's call order

Live observation (modal-realism run): the teacher batched two quizzes in
one turn; the dock showed the SECOND question first, so the answering
order was reversed vs the saved record and the transcript cards.

Cause: findPending scanned parts BACKWARD within a message, so with two
input-available quizzes it picked the last one first. No overwriting was
involved — parts are created in call order on both wire and record; the
scan direction alone reversed the interaction order. (pi executes a
parallel batch with starts + result messages in call order; the
execution-end order is completion-racy, which also allowed a millisecond
dock-subject flip mid-turn.)

Fix A — src/core/pending.ts: parts now scan FORWARD within a message
(messages stay newest-first): the FIRST unanswered question is the dock
subject; the second appears only after the first is answered.

Fix B — src/agent/lh-tool.ts: question tools opt into sequential
execution (pi runs the batch in call order when any called tool opts
in), so tool-input-available chunks always arrive in call order and the
dock subject can never flip mid-turn. Question executes are instant.

Pinned in chat-flow.test.ts: a parallel two-quiz batch asserts wire
order (A's available chunk before B's), part persistence order, and the
dock selection sequence (A first, then B after A is answered).

Gates: tsc, lint, build, 307/307.

src/core/pending.ts

@@ -5,6 +5,13 @@ import { isToolUIPart, type UIMessage } from "ai";
  * blocks the composer — one question at a time, mirroring the teacher's
  * prompt contract. Derived client state; the server enforces nothing.
  *
+ * Scan order: MESSAGES newest-first (a later turn's live question supersedes
+ * an older dangling one), but PARTS forward within a message — the model's
+ * FIRST unanswered question is the dock subject. Parallel question calls
+ * (pi executes them in one batch) are thus answered in the same order they
+ * were asked, saved, and rendered as cards; a backward part scan reversed
+ * that order (the live modal-realism observation).
+ *
  * Job tool parts (research/demo/visual) never pend client-side — the async
  * protocol resolves them server-side with an immediate ack (see
  * contracts/jobs.ts "THE ASYNC PROTOCOL"), so job tools are NOT returned
@@ -26,7 +33,7 @@ export const JOB_TOOLS = ["research", "demo", "visual"] as const;
 function findPending(messages: UIMessage[], names: readonly string[]): PendingToolCall | null {
 	for (let m = messages.length - 1; m >= 0; m--) {
 		const msg = messages[m]!;
-		for (let p = msg.parts.length - 1; p >= 0; p--) {
+		for (let p = 0; p < msg.parts.length; p++) {
 			const part = msg.parts[p]!;
 			if (isToolUIPart(part) && part.state === "input-available") {
 				const tool = part.type.slice("tool-".length);