Policy file survives cross-device mounts and corruption via EBUSY fallback and TOML recovery

What this changes for operators

• Operators running in containers with cross-device mounts no longer hit silent policy-update failures - atomic rename now falls back to copy-then-unlink on EBUSY/EXDEV.
• A corrupted policy TOML is auto-backed-up to .bak and rebuilt from scratch rather than blocking on a syntax error, removing a manual-intervention failure mode.
• Verification path: packages/core/src/policy/config.ts adds the fallback and recovery; persistence.test.ts covers both paths.
• Single operator class (operator persisting policy/permission config), single consequence (policy persistence no longer fails silently).