test(voyage): add Group B (structure) + Group C (annotation export schema) tests [skip-docs]

Step 29 of v4.3 plan — Wave 7:
- Group B (9 tests): DS-token classes (badge--scope-voyage, guide-panel,
  fleet-grid), theme-toggle wiring (data-action, wireThemeToggle,
  localStorage), sidebar-tab keyboard pattern (role=tablist,
  aria-selected, J/K/Esc), anchor-ID format mirror.
- Group C (7 tests, +1 vs original): export-bundle JSON parse, required
  keys, per-annotation field validation, empty-export edge case,
  annotation_digest order-independence, SHA-256 16-hex-char validity
  (SC6 / SC-GAP-3), fixture plan anchor format.
- Fixtures: tests/fixtures/playground/v43-export-bundle.json +
  v43-plan-pre-annotate.md (ANN-0001 + ANN-0002, revision: 0).

Test count: 689 → 705 pass / 0 fail / 2 skipped.

plugins/voyage/tests/fixtures/playground/v43-export-bundle.json

@@ -0,0 +1,25 @@
+{
+  "schema_version": 1,
+  "exported_at": "2026-05-10T18:00:00Z",
+  "target_artifact": "plan",
+  "target_filename": "annotated-plan.md",
+  "annotations": [
+    {
+      "id": "ANN-0001",
+      "target_artifact": "plan",
+      "target_anchor": "step-1-sentinel-touch",
+      "intent": "question",
+      "comment": "Should this sentinel use a deterministic timestamp?",
+      "timestamp": "2026-05-10T18:01:00Z"
+    },
+    {
+      "id": "ANN-0002",
+      "target_artifact": "plan",
+      "target_anchor": "step-2-sentinel-touch-paired",
+      "intent": "fix",
+      "comment": "Step 2 manifest should reference Step 1 in must_contain.",
+      "timestamp": "2026-05-10T18:02:00Z"
+    }
+  ],
+  "annotation_digest": "PLACEHOLDER_OVERWRITTEN_AT_TEST_TIME"
+}