fix(git-sync): subpages round-trips (was {{SUBPAGES}} literal) + exhaustive all-node round-trip test

subpages exported to the literal `{{SUBPAGES}}`, which has no markdown/HTML
inverse, so on re-import it came back as a plain paragraph holding the visible
text "{{SUBPAGES}}" — the embed rendered as that literal string on the page
after a sync (round-trip data loss, seen live). It now emits the schema-matching
`<div data-type="subpages">` like every other embed node, so the schema's
parseHTML rebuilds the subpages node. Also dropped the leaf-atom content-hole
in the subpages renderHTML.

New committed regression coverage:
- packages/git-sync/test/roundtrip-all-nodes.test.ts — exhaustive serialize ->
  deserialize round trip for ALL 40 node/mark types; each asserts the node/mark
  survives and no `{{...}}` literal leaks. This is the test that caught subpages.
- §13.1 gate (git-sync-converter-gate.spec.ts): subpages added to the green
  corpus (round-trips through the REAL server schema).
- Corrected two PR-authored tests that asserted the old {{SUBPAGES}} loss as
  "by design" — they now assert the fixed round trip.

Also folds in review #1679 coverage-gap tests (no prod change): orchestrator
pollTick/enabledSpaces, datasource 3-way merge dispatch, page.repo
last_updated_source provenance SQL.

git-sync vitest 659 (+1 expected-fail), server tsc clean, server specs green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/collaboration/git-sync-converter-gate.spec.ts

@@ -360,6 +360,15 @@ const CORPUS: Record<string, any> = {
       content: [para(text('quote at the end'))],
     },
   ),
+
+  // Atom embeds that carry no inline text: they must round-trip via their
+  // schema-matching HTML (data-type div), NOT a literal that re-imports as plain
+  // text. `subpages` used to export as the literal "{{SUBPAGES}}" and came back
+  // as visible text on the page (red-team round-trip data loss) — this locks it.
+  // editor-ext materializes the `recursive: false` default on import, so the
+  // fixture pre-authors it to sit at the round-trip fixpoint (matches the other
+  // default-materializing fixtures above).
+  'subpages embed': doc({ type: 'subpages', attrs: { recursive: false } }),
 };
 
 describe('git-sync converter §13.1 idempotency gate (editor-ext schema)', () => {