fix: harden related-subquery policy enforcement

[kholdrex]

Summary

Hardens SQL policy enforcement so related-subquery authorization remains fail-closed across compiled and relation-converted queries. The change preserves the original BETWEEN bind cleanup while strengthening policy evidence, adapter-aware identifier handling, and SQL structure scanning against bypasses discovered during adversarial review.

Changes

• extract the shared BETWEEN bind collaborator and preserve false-valued or legacy bind keys
• issue opaque compiler policy contracts and reject direct or replayed policy-bearing queries without matching evidence
• enforce explicit and policy-derived table/column allowlists across related EXISTS and NOT EXISTS occurrences
• harden adapter-aware SQL scanning for quoted identifiers, PostgreSQL TABLE/ONLY, dynamic query functions, literals, comments, and derived-table bypasses
• isolate query state by returning detached copies from public SQL, params, intent, and metrics readers
• document policy-query lifecycle, configuration identity, and compatibility behavior
• expand regression coverage across SQLite, PostgreSQL, and MySQL policy and scanner paths

Test plan

• Full local test suite and style checks passed.
• GitHub Actions package verification and the complete SQLite, PostgreSQL, and MySQL matrix pass on supported Ruby and Active Record versions.