M9 Retention and Garbage Collection Evidence

Coverage: union retention policies, newest-backup protection, pins, tombstones, referenced-object preservation, garbage collection, and reconciliation safety.
Full verification: make check passed with 86 unit/contract, 42 integration, 11 fault, and 17 security tests, plus Ruff, mypy, frontend typecheck, and frontend production build.
Scope: remote source support remains M10 work.