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.

src/core/contracts/chat.ts

@@ -123,6 +123,16 @@ export interface TurnUsage {
  * block renders current node + direct parents + direct children in its own
  * small container, the current node glowing — clearly a fragment, not the
  * whole plan. Clicking either opens the full plan in a popup. */
+/** The learner's plan-acceptance marker (docs/probe-teach-split.md §4): a
+ *  data part riding the accept-click user message. It is THE plan→teach
+ *  phase seam — phaseAt3 reads it from the transcript;
+ *  convertToModelMessages drops data parts, so the teacher only sees the
+ *  message's text; the marker is for derivation and the UI. */
+export interface PlanAcceptedData {
+	/** ISO timestamp of the accept click. */
+	at: string;
+}
+
 export interface PlanBlockData {
 	/** Only "full" survives (the 2026-09-11 pivot): the plan renders IN
 	 *  FLOW once, at authoring. The neighborhood fragments are retired —