fix: core reliability — atomic link writes, conservative metadata, cache synchronization

[rogerdigital]

Summary

Implements part A (A1–A3) of the 1.0.0 release readiness plan (docs/superpowers/plans/2026-09-13-v1-core-reliability.md), three commits in dependency order:

• A1 fix: preserve concurrent edits during link fixes — link fixes now transform content inside vault.process instead of read-then-modify, so an edit committed between read and write is preserved. Deterministic concurrency regression included; failure matrix covers rejected writes and vanished targets.
• A2 fix: withhold link fixes when target metadata is unavailable — a null target cache (not yet indexed) no longer produces a confirmed broken-link finding with an executable fix; it reports unverified/info with evidence.reason: target-metadata-unavailable and no fix action. An empty-but-present cache is still treated as real evidence. COMPARISON_VERSION 3 → 4 isolates older baselines (comment covers both the fix: stop flagging unverifiable and hashless block links as broken #174 block semantics and this change, per the increment-once rule). Adapted to the post-fix: stop flagging unverifiable and hashless block links as broken #174/fix: match Obsidian heading anchors by substituting non-alphanumerics with spaces #176 code structure as recorded in the plan's execution note.
• A3 fix: defer verification until mutation metadata is ready — new MetadataWriteFence correlates each mutation with the exact changed + resolved cache events (and deleted/absence for trash) before verification proceeds; timeouts poison the batch so later preflights are skipped instead of reading stale metadata, with the recovery message the runner surfaces. No CLI schema changes.

Validation

• TDD per item: RED observed before each implementation, GREEN after.
• Full local pipeline per commit: npm run lint && npm run lint:obsidian-warnings && npm run build && npm test && npm pack --dry-run — 949 tests pass.
• Fingerprint inputs for confirmed findings are unchanged; only classification/evidence of the affected findings changed.