Accelerate Rs Search Postgres

1. Postgres Search Indexing

• Create btree_gin during Postgres bootstrap when available.
• Ensure a scoped lexical GIN index for (connector_instance_id, stream, document).
• Keep the existing global document index for compatibility.

2. Runtime Query Safety

• Run Postgres lexical queries with transaction-local max_parallel_workers_per_gather = 0.
• Add a shared bounded fan-out helper for Postgres search work.
• Apply bounded fan-out to lexical owner search snapshot building.
• Apply bounded fan-out to semantic owner search snapshot building.
• Add a bounded Postgres lexical candidate window before rank/snippet computation for broad searches.
• Add short-lived single-flight caching for repeated semantic query vectors.
• Reduce semantic per-connector overscan to the public maximum page size.
• Adapt semantic per-connector overscan to the requested page size.
• Coalesce unfiltered Postgres semantic scope reads per connection.
• Use retained-size estimates to keep small/unknown semantic scopes on exact scans and large scopes on bounded ANN candidate windows.
• Manage bounded hot-source partial HNSW indexes for medium-selectivity semantic sources.

3. Tests

• Add unit coverage for fan-out concurrency and ordering.
• Add/extend Postgres lexical tests to cover scoped index bootstrap and candidate-window lexical query success.
• Add semantic query-vector cache coverage.
• Add semantic overscan and Postgres plan coalescing coverage.
• Add production-dimension Postgres semantic coverage proving ANN candidate retrieval still enforces requested semantic scopes.
• Prove hot-source partial HNSW indexes on the live slow semantic sources before deploy.
• Run targeted search tests.
• Run reference TypeScript.
• Validate OpenSpec strict/all.

4. Live Verification

• Build/deploy in a live-stack window.
• Verify /v1/search?q=the&limit=5 no longer fails with shared-memory errors.
• Verify API benchmark deltas for lexical, semantic, and hybrid search.
• Record residual performance gaps for the next loop.
• Verify API benchmark deltas after broad Postgres semantic retrieval uses ANN candidate windows instead of exact scoped vector scans.