Track directory object high-water marks

[kylebernhardy]

Implements the second reviewable slice of Implement Harper-backed Tantivy Directory mapping and atomic publication #11.

This change:

• adds monotonic chunk and tail high-water metadata to each logical file binding
• reserves chunk ordinals in 64-chunk strides before payload writes, reducing a 1 GiB file from 4,096 binding round trips to 64
• makes reservation, chunk, and publication retries safe when a write applied but its acknowledgment was lost
• advances the directory key format and rejects the prior unreleased format at every storage entry point
• validates published state, physical bounds, and a 1 TiB format safety limit before use
• models RocksDB WAL recovery at atomic batch boundaries and verifies every recoverable prefix across two reservation strides and a non-empty tail publication
• updates the Phase 0 and bounded-reclamation designs with the exact persistence and performance contract

Validation:

• npm run check
• 66 Rust tests
• 35 Node tests plus packed-consumer verification
• Gemini CLI review: approved, no substantive findings
• Claude review: LGTM, no open findings

No production Harper activation or reclamation is introduced by this slice. Those remain gated on the later writer-retirement, bounded-FIFO, and Harper lifecycle work in issue #11.

Comment generated by kAIle (GPT-5)