fix: narrow _fix_blob_seq_ids shim + add repair --mode max-seq-id

The BLOB-seq_id migration shim (PR #664) ran int.from_bytes(..., 'big')
over every BLOB in max_seq_id, including chromadb 1.5.x's own native
format (b'\x11\x11' + 6 ASCII digits). That conversion yields a ~1.23e18
integer that silently suppresses every subsequent embeddings_queue write
for the affected segment (queue filter is seq_id > start), causing
silent drawer-write drops after a 1.5.x upgrade.

Two-part fix:

1. Shim narrowing (mempalace/backends/chroma.py)
   - Drop max_seq_id from the shim loop. chromadb owns that column's
     format; we don't reinterpret it.
   - Defense-in-depth: skip rows in embeddings whose seq_id BLOB has the
     sysdb-10 b'\x11\x11' prefix rather than misconvert.

2. Recovery command (mempalace/repair.py, mempalace/cli.py)
   - mempalace repair --mode max-seq-id [--segment <uuid>]
     [--from-sidecar <path>] [--dry-run] [--yes] [--no-backup]
   - Detects poisoned rows via threshold (seq_id > 2**53).
   - Default heuristic: MAX(embeddings.seq_id) over the collection owning
     the poisoned segment. Matches METADATA max exactly; VECTOR segments
     get a few seq_ids ahead (queue skips an already-indexed window — an
     acceptable loss vs. resetting to 0 and re-processing everything).
   - --from-sidecar copies clean values from a pre-corruption sqlite db.
   - Backs up chroma.sqlite3, closes chroma handles, atomic UPDATEs,
     post-repair verification that raises MaxSeqIdVerificationError if
     any row is still above threshold.

Tests: 8 new in tests/test_repair.py (detection, heuristic, sidecar,
dry-run, segment filter, no-op, backup, rollback-on-verify-failure).
3 new in tests/test_backends.py (max_seq_id untouched by shim,
sysdb-10 prefix skipped in embeddings, legacy big-endian u64 BLOBs
still convert). Full suite: 1103 passed.

CHANGELOG.md

@@ -177,6 +177,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 - Hall detection — routes drawer content to `emotions` / `technical` / `family` / `memory` / `identity` / `consciousness` / `creative` halls, enabling hall-based graph connectivity within wings (#835)
 
 ### Bug Fixes
+- Repair `max_seq_id` corruption caused by `_fix_blob_seq_ids` misinterpreting chromadb 1.5.x's sysdb-10 BLOB format (`b'\x11\x11'` + ASCII digits) as legacy 0.6.x big-endian BLOBs. The shim now skips the `max_seq_id` table entirely and guards the `embeddings` branch with a prefix check. New subcommand `mempalace repair --mode max-seq-id [--from-sidecar <path>]` restores affected palaces. Fixes silent drawer-write drops that began after chromadb 1.5.x upgrades on palaces that still had BLOB-typed `max_seq_id` rows at migration time.
 - Set `hnsw:space=cosine` metadata on all collection creation sites — fixes broken similarity scoring under ChromaDB's default L2 distance (#807, #218)
 - File-level locking prevents duplicate drawers when agents mine the same file concurrently (#784, #826)
 - Hybrid closet+drawer retrieval — closets boost ranking, never gate results (#795)