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-plan.md

@@ -0,0 +1,64 @@
+---
+plan_version: 1.7
+profile: balanced
+---
+
+# Demo plan 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 `validatePlan`).
+
+## Context
+
+A minimal plan with two steps. Each step has a Manifest block so
+`plan-validator --strict` accepts the file.
+
+## Implementation Plan
+
+### Step 1: Touch a sentinel file
+
+- **Files:** `tmp/sentinel-1.txt` (new)
+- **Changes:** Create the sentinel file with the literal content "step-1".
+- **Reuses:** none.
+- **Test first:** none — sentinel-only step.
+- **Verify:** `test -f tmp/sentinel-1.txt`
+- **On failure:** revert.
+- **Checkpoint:** `git commit -m "chore: sentinel step 1"`
+- **Manifest:**
+  ```yaml
+  manifest:
+    expected_paths:
+      - tmp/sentinel-1.txt
+    min_file_count: 1
+    commit_message_pattern: "^chore: sentinel step 1"
+    bash_syntax_check: []
+    forbidden_paths: []
+    must_contain: []
+  ```
+
+### Step 2: Touch a second sentinel file
+
+- **Files:** `tmp/sentinel-2.txt` (new)
+- **Changes:** Create the sentinel file with the literal content "step-2".
+- **Reuses:** none.
+- **Test first:** none.
+- **Verify:** `test -f tmp/sentinel-2.txt`
+- **On failure:** revert.
+- **Checkpoint:** `git commit -m "chore: sentinel step 2"`
+- **Manifest:**
+  ```yaml
+  manifest:
+    expected_paths:
+      - tmp/sentinel-2.txt
+    min_file_count: 1
+    commit_message_pattern: "^chore: sentinel step 2"
+    bash_syntax_check: []
+    forbidden_paths: []
+    must_contain: []
+  ```
+
+## Verification
+
+- `npm test` passes.
+- Both sentinel files exist.