test(voyage): add annotation roundtrip + rollback + source_annotations integration — v4.2 Step 7

Implements SC2/SC3/SC5b/SC7 + additive-field invariant for the v4.2
annotation pipeline:

Fixtures (tests/fixtures/annotation/):
  - annotation-brief.md           — brief-validator-clean fixture
  - annotation-plan.md            — plan-validator-clean (2 steps)
  - annotation-review.md          — review-validator-clean
  - annotation-plan-large.md      — 51 steps (SC3 scale fixture)

Integration tests:
  - tests/integration/annotation-roundtrip.test.mjs — 7 cases:
    SC2 byte-identical empty round-trip across brief/plan/review,
    SC3 scale (51 steps + 100 anchors) round-trip,
    SC7 parseAnchors(stripAnchors(addAnchors(...))) === [] per target.
  - tests/integration/schema-rollback.test.mjs — 4 cases:
    SC5b validator-FAIL -> revisionGuard rolls back byte-identical
    (sha256 invariant) for brief/plan/review + cross-target sweep.
    .local.bak deleted on rollback path (validator-PASS path tested
    in lib/util/revision-guard tests).
  - tests/lib/source-annotations.test.mjs — 6 cases mirroring
    tests/lib/source-findings.test.mjs additive-field pattern: each
    validator (brief/plan/review) accepts source_annotations as
    additive-optional, parser extracts as array of dicts, entries
    conform to documented shape, baseline forward-compat (artifacts
    without source_annotations still validate).

Verify: node --test tests/integration/annotation-roundtrip.test.mjs
       tests/integration/schema-rollback.test.mjs
       tests/lib/source-annotations.test.mjs -> 17 pass / 0 fail.
Full npm test: 577 pass / 0 fail / 2 skipped (Docker).

Refs plan.md Step 7 + plan-critic M4 + plan-critic B4.

plugins/voyage/tests/fixtures/annotation/annotation-brief.md

@@ -0,0 +1,34 @@
+---
+type: trekbrief
+brief_version: "1.0"
+task: Demo task for annotation round-trip fixture
+slug: annotation-brief-demo
+research_topics: 0
+research_status: complete
+---
+
+# Demo brief for annotation round-trip
+
+This fixture is used by `tests/integration/annotation-roundtrip.test.mjs`
+to verify SC2 (byte-identical empty-anchor round-trip) and SC7 (per-target
+isolation against `validateBrief`).
+
+It carries no anchors. The round-trip test runs:
+`stripAnchors(addAnchors(body, [])) === body`.
+
+## Intent
+
+Provide a minimal brief that validates against `brief-validator.mjs` so
+the round-trip integration test has a real artifact to revise.
+
+## Goal
+
+The brief should validate cleanly (no errors, no warnings) and contain
+enough body text that adding an anchor and stripping it back is a
+non-trivial operation.
+
+## Success Criteria
+
+- File parses via `parseDocument`.
+- `validateBrief` returns `valid: true`.
+- `stripAnchors(addAnchors(body, []))` is byte-identical to body.