Plan blocks in chat: full DAG borderless in the flow, fragment + glow, live popup (wave-4 item 3)

- components/plan-block.tsx: hand-built SVG (user decision) — deterministic
  layered layout (Kahn topo + longest-path depth, rows centered, authoring
  tie-break), bezier edges, the retired panel's status color language.
- PlanBlock renders data-plan-block parts AS CARRIED (frozen snapshots,
  never re-derived): kind full = borderless, viewBox = layout extent so
  nodes hug the column edges, natural height; kind neighborhood = own
  small labeled 'Plan fragment' container with the current node glowing
  (pulsing primary halo).
- PlanPopup: full-screen overlay with the LIVE plan — latest tree from the
  transcript, statuses derived at click time via derivePlanStatuses
  (contracts/plan.ts). Esc / backdrop / X close; drag pan, wheel
  zoom-to-cursor, zoom buttons; fit-floor semantics harvested from the
  retired tree panel (zoom floor = the fit, pan locks at it).
- chat.tsx: data-plan-block case in the part switch + popup mount.
- src/core/ui-message.ts: TeacherUIDataParts gains "plan-block": PlanBlockData
  (additive, type-only — lane touch flagged to master).
- tests/render-plan-block.test.tsx: pure layout (layering, centering,
  dangling-edge tolerance), wrap, frozen-snapshot render, live derivation.

src/core/ui-message.ts

@@ -1,5 +1,5 @@
 import type { UIMessage } from "ai";
-import type { DemoToolInput, ResearchToolInput, SubagentArtifactData, VisualToolInput } from "./contracts.ts";
+import type { DemoToolInput, PlanBlockData, ResearchToolInput, SubagentArtifactData, VisualToolInput } from "./contracts.ts";
 import type { AskToolInput, QuestionAnswerData, QuizToolInput } from "./question-core.ts";
 
 /**
@@ -24,6 +24,8 @@ export type TeacherUITools = {
 
 export type TeacherUIDataParts = {
 	"subagent-artifact": SubagentArtifactData;
+	/** Plan blocks (wave 4): frozen plan snapshots rendered in the chat flow. */
+	"plan-block": PlanBlockData;
 };
 
 export type TeacherUIMessage = UIMessage<unknown, TeacherUIDataParts, TeacherUITools>;