#8348 improvement(knowledge): serve date tag filters from an index

Summary

• Date tag filters compiled to col::date <op> d::date, which no btree on the timestamp column can serve. Every date filter walked past its slot index, so #8332 dropped those never-used indexes. This makes date filtering index-backed instead
• Date filters now compile to half-open ranges on the raw timestamp column, e.g. eq d → col >= d AND col < d + 1 day. Each range selects exactly the rows the calendar-day comparison did: the slots have no time zone, and a NULL tag still matches nothing
• One builder, buildDateTagCondition, is shared by search tag filters and the document list filter. Each keeps its existing fallback for unknown operators
• New migration re-creates {emb,doc}_date1_idx / date2_idx concurrently

Type of Change

• Improvement

Testing

• Real-Postgres equivalence test: every operator, every day boundary (23:59:59.999999, midnight), NULL, and equal-endpoint and multi-day between, under three session time zones, compared against the old ::date predicate evaluated on the same rows. Five boundary mutations of the builder each turn it red
• Isolated EXPLAIN test: selective eq, between, gte, and lt date filters use the date index from both builders. It fails on the pre-change builders
• Knowledge search A/B on a synthetic 732k-chunk fixture, with identical results in every shape:
  • a single-day filter no longer seq-scans embedding in the vector leg's short-result rescue probe (55–61 ms → under 1 ms)
  • vector and hybrid date searches run 19–25% faster end to end
  • hybrid keyword ranking with a date filter drops from ~30 → ~10 ms
  • tags-only single-day filters add ~1 ms
• kb-block-search, filtered-search, workspace-kb-document-access, search-index-policy, type-check, lint, check:audits, and check:migrations origin/staging pass
• Migration number is renumbered at merge time, since other open PRs also add migrations

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)