fix: remove retrieval reads of dropped namespace statistics

[suguanYang]

Summary

Make retrieval compatible with the namespace-statistics removal in PR #366.

Changes

• Remove classic-route reads from retrieval_namespace_stats and retrieval_namespace_token_stats.
• Remove the map-nav synchronous scorer's query against namespace token statistics.
• Keep scoring based on the persisted per-revision map-unit index inputs:
  • document_map_units
  • document_map_unit_tokens
  • document_map_unit_indexes
• Retain the existing unit-count-weighted revision average IDF fallback, which is stored in the map-unit index and remains available after migration 9f0a1b2c3d4e.

After this change, applying PR #366's drop migration will not leave retrieval with references to removed tables.

Validation

• Retrieval contract suite: 26 passed.
• Ruff: passed.
• Pyright: 0 errors.
• Repository search confirms no runtime retrieval references to retrieval_namespace_stats or retrieval_namespace_token_stats.

No production database writes were performed.