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/ui-message.ts

@@ -1,6 +1,7 @@
 import type { UIMessage } from "ai";
 import type {
 	DemoToolInput,
+	PlanAcceptedData,
 	PlanBlockData,
 	ProbeDoneInput,
 	ProbeDoneResult,
@@ -37,6 +38,9 @@ export type TeacherUIDataParts = {
 	"subagent-artifact": SubagentArtifactData;
 	/** Plan blocks (wave 4): frozen plan snapshots rendered in the chat flow. */
 	"plan-block": PlanBlockData;
+	/** The plan-acceptance marker: the accept button's user message carries
+	 *  it — the plan→teach phase seam (docs/probe-teach-split.md §4). */
+	"plan-accepted": PlanAcceptedData;
 };
 
 export type TeacherUIMessage = UIMessage<unknown, TeacherUIDataParts, TeacherUITools>;