WS-2 items 4+5: draw structure on canvas + preserve N-deep tree on save

Round-trip fix (item 5): serializeCanvasShapes no longer re-derives the question
tree from boundary pairs (which flattened 3-deep → 2-tier). The whole tree is now
carried as 'part' shapes that hold parentId + isContainer + depth; save preserves
the real parent_id chains (persisted parent wins; geometry is only a fallback for
manually-drawn shapes). Boundary-pair synthesis is kept but attaches to a covering
container box when one exists. New test locks a 3-deep Q→part→subpart round-trip.

Draw structure (item 4): shapesFromTemplate now draws containers too (were skipped),
each with nesting depth. examCanvasShapes renders container frames (transparent,
depth-hued border), PRINTS MARKS on part/container boxes (◆N pill; were never
drawn), and NAMES context figures (figure name as label + description tooltip +
'→ Q' tether pill; was an anonymous purple box). Context name/description round-trip
via exam_response_areas.meta.

Threads new shape props through the tldraw util + setup page + types. Adds meta to
ExamResponseArea + replace payload. tsc: no new errors; 7/7 model tests pass.

Co-Authored-By: Claude Opus 4.8 <[email protected]>
Claude-Session: https://claude.ai/code/session_01GruxHXxfdp4kZCgAMVFgvV

src/types/exam.types.ts

@@ -91,6 +91,10 @@ export interface ExamQuestion {
   spec_ref: string | null;
   bounds?: Record<string, number> | null;
   page?: number | null;
+  /** Analyse contract v2 (migration 78): primary command verb for a leaf part. */
+  command_word?: string | null;
+  /** Analyse contract v2 (migration 78): stem prose for the outline view. */
+  preamble?: Record<string, unknown> | null;
   source: ExamTemplateSource;
   confirmed: boolean;
   confidence: number | null;
@@ -116,6 +120,8 @@ export interface ExamResponseArea {
   kind: ExamResponseAreaKind;
   response_form: string | null;
   context_type?: string | null;
+  /** Rich recognition payload (migration 75): figure name/description, OMR box geometry, unit/quantity, table… */
+  meta?: Record<string, unknown> | null;
   source: ExamTemplateSource;
   confirmed: boolean;
   confidence: number | null;
@@ -197,6 +203,7 @@ export interface TemplateReplacePayload {
     kind: ExamResponseArea['kind'];
     response_form?: string | null;
     context_type?: string | null;
+    meta?: Record<string, unknown> | null;
     source?: 'manual' | 'ai';
     confirmed?: boolean;
     confidence?: number | null;