feat: the plan verdict gate — acceptance is the plan→teach seam

The accept-plan UI lands (docs/probe-teach-split.md §4, formerly the
interim rule): presenting the plan no longer moves the phase — the
learner's ACCEPT does. While the presented plan awaits its verdict, the
composer's send square becomes two stacked buttons (green Accept plan /
red Decline); typed text rides the verdict as his note, Enter keeps
sending plain side questions (phase unmoved), decline asks for a
revision and the buttons stay.

The seam: accept sends a user message carrying a data-plan-accepted
part (PlanAcceptedData in contracts/chat.ts, TeacherUIDataParts
'plan-accepted'); phaseAt3 reads it as the plan→teach event (plan_init
demoted to presentation-only); convertToModelMessages drops data parts,
so the teacher sees only the text. Mid-lesson rewrites need no fresh
verdict — acceptance happens once, at the first presentation.

The teacher's planning prompt now ends the turn at presentation and
waits for the verdict (never teaches into an unaccepted plan).

Gates: tsc clean, 304/304 (phases tests rewritten to the acceptance
seam incl. decline/side-question/failed-init cases; composer verdict
render tests), lint, build. Live GLM smoke ALL PASS: phase pinned at
'plan' after plan_init, presentation turn carried NO check, accept
click flipped the summary to 'teach', then tagged checks moved the map
(current=rates, visited=composite); zero console errors.

components/chat.tsx

@@ -21,7 +21,7 @@ import { SubagentSteerCard } from "@/components/artifacts/steer-card";
 import { Composer } from "@/components/composer";
 import { JobsContext } from "@/components/jobs-context";
 import { StatsBar } from "@/components/stats-bar";
-import { PlanBlock, PlanPopup } from "@/components/plan-block";
+import { PlanBlock, PlanPopup, latestTree } from "@/components/plan-block";
 import { PlanWindow } from "@/components/plan-window";
 import type { PlanBlockData } from "@/src/core";
 import { QuestionDock } from "@/components/question-dock";
@@ -181,6 +181,37 @@ export function Chat({
 	// The live plan popup (wave 4): opened by any transcript plan block.
 	const [planOpen, setPlanOpen] = useState(false);
 
+	// ── The plan verdict gate (docs/probe-teach-split.md §4) ──
+	// The plan has been presented (a tree exists) and the session still sits in
+	// the planning phase → the composer's send square becomes the stacked
+	// Accept / Decline buttons. Acceptance is the explicit plan→teach seam
+	// (a data-plan-accepted part on the message); decline asks for revision.
+	const planVerdict = useMemo(
+		() => phaseAt3(messages) === "plan" && latestTree(messages) !== null,
+		[messages],
+	);
+	const sendVerdict = useCallback(
+		(accepted: boolean) => {
+			const note = text.trim();
+			setText("");
+			if (accepted) {
+				void sendMessage({
+					parts: [
+						{ type: "text", text: note || "The plan is accepted — begin teaching." },
+						{
+							type: "data-plan-accepted",
+							id: `pa_${Date.now().toString(36)}_${crypto.randomUUID().slice(0, 8)}`,
+							data: { at: new Date().toISOString() },
+						},
+					],
+				});
+			} else {
+				void sendMessage({ text: note || "I'd like the plan revised before we begin." });
+			}
+		},
+		[text, sendMessage],
+	);
+
 	// ── Session usage (wave-4 stats bar) ──
 	// The money channel: the record's turns log, folded client-side by
 	// core/prices.ts. Seeded once on mount (the page doesn't carry turns),
@@ -489,6 +520,14 @@ export function Chat({
 									onSend={onSend}
 									onStop={() => stop()}
 									canSend={canSend}
+									verdict={
+										planVerdict
+											? {
+													onAccept: () => sendVerdict(true),
+													onDecline: () => sendVerdict(false),
+												}
+											: undefined
+									}
 								/>
 							}
 							onSubmitAnswer={(output) => {