test(sync): implement test-strategy Phase 1-2 (pure unit/golden/property), +102 tests

Work through test-strategy-report.md, high-ROI no-refactor subset (no regen).

- R-Infra: vitest resolve.alias docmost-client -> packages/docmost-client/src
  (fixes the dist-vs-src coverage artifact: canonicalize 0% -> real)
- R-Cfg-1: export parseArgs + tests
- canonicalize: align family / comment.resolved kept / link non-default +
  fixpoint & docsCanonicallyEqual reflexive/symmetric properties (0 -> 100%)
- markdown-converter golden matrix: columns/embed/audio/pdf, drawio data-align
  rule, inline-mark matrix, textAlign, escaping idempotence, table sanitization
  (61 -> 79%)
- schema parse-closures via generateJSON (TextStyle/comment/mention/Highlight/Column)
- node-ops (immutability, table edge cases, makeFreshId property), transforms
  (setCalloutRange/insertMarkerAfter/commentsToFootnotes + renumber property)
- stabilize normalize-on-write fixpoint (0 -> 100%); diff coarse-fallback;
  client-utils; firstDivergence; corpus fixtures details/columns/mention
- 593 -> 695 green; build clean; corpus STABLE

Deferred (Phase 3-4, refactor-gated): pull/collab/client-REST/git-merge integration.

test/fixtures/corpus/10-mention-in-heading.json

@@ -0,0 +1,13 @@
+{
+  "type": "doc",
+  "content": [
+    {
+      "type": "heading",
+      "attrs": { "level": 2 },
+      "content": [
+        { "type": "text", "text": "Notes for " },
+        { "type": "mention", "attrs": { "id": "m-2", "label": "Bob", "entityType": "user", "entityId": "u-2", "slugId": "s-2", "creatorId": "c-2" } }
+      ]
+    }
+  ]
+}