Module suspend_ops 

Suspend / deliver_signal / claim_resumed / observe_signals SQL bodies.

Wave 4d follow-up. Builds on PR #238’s primitives:

• crate::signal::hmac_sign / crate::signal::hmac_verify
• crate::signal::SERIALIZABLE_RETRY_BUDGET + crate::signal::is_retryable_serialization
• crate::suspend::evaluate — composite-condition evaluator

HMAC signing, retry looping, and composite evaluation are never re-implemented here.

Isolation

suspend + deliver_signal run at SERIALIZABLE with a 3-attempt retry budget (Q11). On retry exhaustion they surface Contention(RetryExhausted). claim_resumed_execution + observe_signals stay at READ COMMITTED (row-level FOR UPDATE for the former, read-only for the latter).