perf: avoid repeated section scans in map discovery

[suguanYang]

Summary

Fixes the production statement timeout in map-unit discovery for large namespaces.

The frequency and index-stat queries previously reused the full scoped_units CTE, including a document_sections join and all section paths. For a 644-document namespace this repeated scan over 42,794 map units and caused asyncpg.exceptions.QueryCanceledError under the 30-second statement timeout.

This change adds a lightweight map-unit-ID CTE for frequency and index-stat lookups. The full section join remains only where section-path filtering is required.

Production evidence

Against the repaired read-only production namespace:

• 644 active documents
• 42,794 indexed map units
• 644 matching manifests and indexes
• The full scoped query took about 12.4 seconds; the repeated frequency query was the statement canceled in production.

Validation

• Map-unit and classic retrieval contract tests: 6 passed.
• Ruff: passed.

No production writes were performed.