feat(voyage): add markdown-write.mjs + revision-guard.mjs + forward-compat policy comments — v4.2 Step 1
- lib/util/markdown-write.mjs: serializeFrontmatter (subset matches frontmatter.mjs parser), atomicWriteMarkdown (single tmp+rename, body bytes verbatim), readAndUpdate (read+mutate+write).
- lib/util/revision-guard.mjs: revisionGuard(path, mutator, validator) — backup -> mutate -> validate -> restore-on-fail. Extracted from /trekrevise prompt so rollback can be unit-tested.
- 12 tests for markdown-write, including 6-key source_annotations round-trip + walk-all-fixtures regression.
- 6 tests for revision-guard: applied/rolled-back/mutator-failed/sha256 stability/pre-existing-bak abort.
- Forward-compat policy comments in 3 validators (brief/plan/review) — non-functional pin against future strict-key refactors.
Pass: 508/510 (was 490; +18 net from v4.2 Step 1, 2 skipped Docker)
Kjell Tore Guttormsen
dcf0c7ad02