Replace validate_assignment=True with frozen=True on Invariant,
InvariantViolation, InvariantEnforcementRecord, and InvariantSet models
to enforce the value-object immutability contract required by the spec.
Redesign InvariantService.remove_invariant() to use model_copy(update=
{'active': False}) instead of direct field mutation, since frozen models
raise ValidationError on assignment.
Fix invariant_reconciliation_actor_steps.py step that mutated
inv.non_overridable = True to construct the Invariant directly with
non_overridable=True at creation time.
Add BDD scenarios covering:
- Immutability contract: mutation raises error on all three models
- Hashability: frozen Pydantic v2 models are hashable
- Soft-delete copy-and-replace: removed invariant is a new object
All 266 scenarios pass. Lint and typecheck pass.
ISSUES CLOSED: #3116
freemo