feat(escrow): guard release/refund paths with a circuit breaker

[Favouromale]

Summary

Adds a circuit breaker to the escrow contract's highest-value flows so an authorized guardian can halt fund movement instantly if an anomaly or exploit is detected, without blocking read-only queries.

closes #6

• TimeLockedEscrowContract::release_escrow, batch_release, and refund_escrow now check the breaker first and revert with the new EscrowError::CircuitBreakerTripped when tripped.
• trip_circuit_breaker(caller, reason) / reset_circuit_breaker(caller) are gated to the contract admin or an optional dedicated breaker guardian address (set via the new admin-only set_breaker_guardian), mirroring the trip/reset access-control pattern used by the existing circuit_breaker.rs / submission_pause.rs modules elsewhere in this repo.
• Read-only queries (get_escrow, get_challenge, get_dispute, get_accrued_fees, plus the new is_circuit_breaker_tripped / get_circuit_breaker_state) remain available regardless of breaker state.
• New CircuitBreakerTripped / CircuitBreakerReset events for auditability, and a CircuitBreakerState record (reason, actor, timestamp) exposed via get_circuit_breaker_state.
• No changes to unrelated existing behavior (e.g. set_emergency_pause / emergency_recover are untouched).

Testing / validation performed

All run locally against the workspace at HEAD of this branch:

• cargo fmt --all -- --check — clean
• cargo clippy --all-targets --all-features -- -D warnings — clean (whole workspace)
• cargo build --release --target wasm32-unknown-unknown — succeeds
• cargo test — all 18 test binaries pass (including the 3 new circuit-breaker tests), 0 failures
• cargo test --test escrow_contract_fuzz -p escrow-contract (FUZZ_ITERATIONS=200, as CI runs it) — passes

New tests added in escrow_contract/tests/escrow_contract.test.rs:

• circuit_breaker_blocks_release_and_refund_when_tripped — tripped breaker rejects release_escrow/refund_escrow with CircuitBreakerTripped, while get_escrow/get_circuit_breaker_state keep working.
• circuit_breaker_reset_restores_release_and_refund — reset clears the trip and release proceeds normally.
• circuit_breaker_only_authorized_role_can_trip_or_reset — an unauthorized caller is rejected; a delegated guardian (set via set_breaker_guardian) can trip/reset, a stranger cannot.

Issue

#6