Prevent Harper full-text index key collisions

[kylebernhardy]

Introduces an unambiguous, versioned key encoding and durable format validation for Harper-backed Tantivy directories. It rejects the unreleased delimiter-based prototype instead of silently opening a second empty keyspace and records the bounded reclamation delivery sequence for the remaining work in Implement Harper-backed Tantivy Directory mapping and atomic publication #11.

For the human reviewer

1. Confirm the storage-incarnation boundary: after a clean initial sentinel probe, the directory trusts a valid marker and uses a single-marker-read empty-state cache, assuming Harper restore replaces a closed generation rather than merging older prototype bytes into a live namespace. If Harper permits merge-style restore, this slice needs an explicit create-versus-open contract before the format is released; otherwise the chosen steady state avoids repeated host round trips.

Verification

• npm run format:check — passed.
• npm run lint — TypeScript and Clippy passed with warnings denied.
• npm test — 59 Rust tests, 35 Node tests, and the packed-package consumer test passed; the three opt-in historical rocksdb-js lease tests remained skipped as expected.
• Read-only Claude and actual Gemini CLI final reviews completed. Gemini reported no actionable findings; Claude retained only the Harper restore-boundary judgment above and the already-documented slice-2 requirement to update the binding equality guard when high-water fields land.

Comment generated by kAIle (GPT-5)

Complexity: complicated

_{Review-Coverage: authored=codex; ran=claude; declined=gemini,cursor-grok,cursor-composer,domain; rounds=4 @ afb6bcc}

_{Human-Review-Need: 3 @ afb6bcc}