Phase 4 Stack 2 — Core Override Enforcement — CLOSED.

Canonical override schema is hash-bound and enforced.
Override registry validation executes unconditionally at CI and runtime.
Boot manifest validation cannot be bypassed.
Exit 5 (Registry Missing) and Exit 6 (Registry Invalid) trigger hard runtime refusal.
Override entries now require enumerated reason codes and expiration metadata to prevent governance creep.

Stack 2 enforcement is mechanically complete.
No soft enforcement surfaces remain.

Proceeding to Phase 5 Activation Spine.