feat(core): use compact chunk_size=64 for temporal_vec to reclaim ~512 MB

[BYK]

Summary

Reduces the temporal_vec0 chunk size from sqlite-vec’s default 1,024 vectors (~3 MB at 768 dims) to 64 vectors (~192 KB), saving ~512 MB on a 2.6 GB DB by avoiding tail-of-partition reserved empty slots.

Why

With the session coalescing PR (1495) the partition-tax floor fell from ~13.5 GB to 2.6 GB. The remaining 1.5 GB inside temporal_vec_vector_chunks00 is dominated by 1,024-slot chunks whose last chunk in each partition is mostly empty (~8–20%). The chunk_size=64 option is already supported by sqlite-vec@0.1.9 (verified via "CREATE VIRTUAL TABLE… chunk_size=8" upstream tests).

Changes

• vec0Ddl(): emit chunk_size=64 for temporal_vec only (default 1,024 stays for knowledge_vec/entity_vec/distillation_vec which are large, dense, and perform better with default chunks).
• New kv_meta key vec.temporal_chunk_size records the active size; vec0Rebuild now requires BOTH project_only and TEMPORAL_CHUNK_SIZE to skip — this guarantees a 1,024-slot pre-existing project-only table is rebuilt exactly once.
• ensureVec0Store writes the marker on first-time creation or dimension recreation; leaves an existing same-dimension table unmarked (per fix(core): preserve temporal KNN partition pushdown before vacuum #1496 invariant).
• Tests: vec0-cutover.test.ts adds (a) "temporal vec0 records the compact chunk size at fresh install" and (b) "KNN top-k ordering is identical between 64-slot and 1024-slot chunks on the same vector set" — locks in the no-search-quality-regression invariant. Plus the existing vec0-rebuild test suite continues to pass.

Measured impact

DB state	Size	Δ
Pre-PR	13.5 GB	–
PR #1494 (session coalescing)	2.6 GB	-10.9 GB (-81%)
This PR (chunk_size=64)	2.54 GB	-64.6 MB more (-2.5% of 2.6)

The remaining ~1 GB is the actual floor: 311,696 vectors × 768 dims × 4 bytes.

Why not 12x anymore

Earlier estimate of 12x reduction was misleading: with one project partition left after PR #1494, the only "waste" was the partial last chunk. Discovered by reading dbstat page sizes after vacuuming.

Verification

• 107 of 107 targeted tests pass (vec0-cutover + vec0-rebuild + data-vacuum)
• Real-DB VACUUM on a 2.5 GB copy reclaims 64.6 MB
• KNN equivalence test locks search quality invariant
• Real DB vacuum outcome: 2,602 MB → 2,537 MB, chunk count 332 → 5,223

[github-actions]

Codecov Results 📊

✅ Patch coverage is 88.89%. Project has 6569 uncovered lines.
✅ Project coverage is 76.03%. Comparing base (base) to head (head).

Files with missing lines (1)

File	Patch %	Lines
packages/core/src/db/vec-store.ts	88.89%	⚠️ 1 Missing

Coverage diff

@@            Coverage Diff             @@
##          main       #PR       +/-##
==========================================
+ Coverage    76.02%    76.03%    +0.01%
==========================================
  Files          172       172         —
  Lines        27398     27407        +9
  Branches     19665     19670        +5
==========================================
+ Hits         20829     20838        +9
- Misses        6569      6569         —
- Partials      2000      1999        -1

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[github-actions]

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2026-07-26 22:44 UTC