feat(voyage): add anchor-parser.mjs with placement validation — v4.2 Step 3

lib/parsers/anchor-parser.mjs (~190 LoC):
- parseAnchors(md) -> Anchor[] (id, target, line, snippet?, intent?)
- addAnchors(md, anchors) -> md_with_anchors
- stripAnchors(md_with_anchors) -> md (byte-identical)
- validateAnchorPlacement(md, anchors) -> errors for list-item / fenced-block / indent

Format: <!-- voyage:anchor id="ANN-NNNN" target="<slug>" line="<N>" -->
Block-level only, on its own line (col 0), blank-line separation.

Test fixture annotation-example.md with single ANN-0001 anchor — referenced by SC12 quickstart.
14 tests pass (parseAnchors, addAnchors, stripAnchors, validateAnchorPlacement).

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

@@ -0,0 +1,27 @@
+---
+type: trekplan-fixture
+plan_version: "1.7"
+created: 2026-05-09
+slug: annotation-example
+---
+
+# Sample plan with one anchor
+
+This fixture is referenced by `docs/annotation-quickstart.md` and the SC12
+machine-proxy verification (`parseAnchors` exits 0).
+
+## Section A
+
+A normal paragraph in section A.
+
+<!-- voyage:anchor id="ANN-0001" target="section-b" line="20" intent="change" -->
+
+## Section B
+
+A paragraph in section B that the anchor above refers to. The anchor is
+placed on its own line with a blank line above and below — the canonical
+v4.2 placement disipline.
+
+## Section C
+
+Another paragraph.