Add bounded reclamation for Harper-backed Tantivy directories

[kylebernhardy]

Adds a synchronous, budgeted reclamation core for the experimental Harper-backed Tantivy directory. Retired objects drain through durable per-sequence progress while live file handles remain pinned, and terminal corruption is surfaced for generation rebuild rather than silently skipped; review most closely the monotonic tail-snapshot boundary and its deterministic concurrent-enqueue proof.

For the human reviewer

No open judgment calls — independent Claude review converged at efdbf69, and direct Gemini review verified the corrected cursor and tail-hint concurrency paths.

Verification

• cargo test --locked --all-features: 106 tests passed, including real Tantivy merge, garbage collection, physical drain, crash recovery, and reopen; documentation tests passed.
• cargo clippy --locked --all-features --all-targets -- -D warnings: passed.
• npm run test:node: 35 Node/native tests and packed-package verification passed; three pre-existing rocksdb-js opt-in tests skipped because no feature-enabled checkout was configured.
• cargo bench --locked --features phase0 --bench kv_directory -- --smoke --revision reclamation-consumer-final: passed, including physical reclamation of deleted 4 KiB directory objects.

Complexity: complicated

Comment generated by kAIle (GPT-5)

_{Review-Coverage: authored=codex; ran=claude; declined=gemini,cursor-grok,cursor-composer,domain; rounds=4 @ efdbf69}

_{Human-Review-Need: 3 @ efdbf69}