test: add result-level ANN plan validation above k threshold

[lindixu6-hash]

Summary

Follow-up from #259 review feedback by @NaNomicon: the existing SQL-text assertion in turso-vector-search.test.ts only verifies the query contains CROSS JOIN and string ordering, which would pass even if the planner silently reordered and search degraded.

This adds a behavior-level test:

• Seeds 200 real memories across two container tags (150 target + 50 other), well above the k=128 ANN threshold for tagged searches.
• The target memory at index 0 has the query vector itself → must rank first with similarity > 0.9.
• Verifies no other-container-tag memories leak into results (indices 150–199 must not appear).
• Verifies result count respects the requested limit.

Testing

• bun test tests/turso-vector-search.test.ts — 3/3 pass (existing 2 + new 1)
• tsc --noEmit clean

[lindixu6-hash]

Reviewer quick guide: This is a test-only PR (no source changes). It adds one integration test that seeds 200 real memories across two container tags (above the k=128 ANN threshold) and verifies:

1. The closest target-tag memory ranks first (similarity > 0.9)
2. No other-tag memories leak into results
3. Result count respects the limit

This addresses @NaNomicon's follow-up from #259 review: the existing SQL-text test only checks for CROSS JOIN in the query string, which would pass even if the planner reordered. This test validates actual ANN behavior at the result level.