fuzz: cover chanmon force-close settlement

Fold the mempool follow-up into the force-close fuzzing layer so
this branch has one commit for settlement coverage.

Keep relay and mining opcodes from the mempool model while adding
explicit and timeout-driven close tracking, holder signer unblocks,
and cleanup that drives on-chain claims. Skip stale PaymentClaimable
events once their claim_deadline has passed so the harness does not
call claim_funds outside the LDK API contract.

This leaves PaymentClaimBuffer as an accounting failure after a live
claim attempt instead of clearing the pending settlement obligation.

Author: joostjager