circuit-breaker: add a fuzz target combining freeze state with a composed consumer's transfer path

[Idaonoli]

Scope

Circuit-breaker's own contract-level fuzzing is already tracked separately, but nothing fuzzes the composed scenario — a consumer contract (like examples/circuit-breaker-consumer, once it exists) checking is_frozen() mid-sequence of its own state changes, to catch integration-level bugs a single-contract fuzz harness can't see.

Acceptance criteria

• Add a fuzz/property test at the composition level: a consumer contract's operations interleaved with random freeze/unfreeze calls, asserting the consumer never allows a gated operation through while frozen