Invariant IDs

Every invariant failure MUST emit an ID. Defaults use INV-000 but should be replaced with specific IDs over time.

• INV-000: unspecified invariant violation (temporary default)
• INV-010: execution contract mismatch
• INV-011: backend capability refusal
• INV-020: missing randomness for ND execution
• INV-021: budget exhaustion triggered
• INV-030: plan fingerprint mismatch
• INV-040: deterministic replay divergence

Each ID MUST appear in exception text and MUST be covered by at least one test.