fuzz: add chanmon holder signer fuzz ops

[joostjager]

Allow chanmon consistency inputs to block and later unblock holder-side signing operations. This lets focused force-close fuzzing reuse the signer-op machinery without carrying the larger mining and settlement model.

I split this out ahead of the broader force-close fuzzing work so we can review the signer-op modeling independently. The main question is whether these holder-side unblock actions are worth spending dedicated chanmon consistency opcodes on, or whether we should keep the opcode surface smaller and rely on final cleanup to re-enable them.

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

I've thoroughly reviewed the entire PR diff, examining:

1. The expanded SUPPORTED_SIGNER_OPS array (4 -> 6 elements)
2. The new enable_holder_signer_ops() helper method
3. The new opcodes 0xe4-0xe6 in the fuzz dispatch
4. The settle_all finalization changes adding monitor.signer_unblocked(None) calls

I verified that:

• SignHolderCommitment is only used in chain monitor paths (chain/package.rs:668, chain/channelmonitor.rs:5393), NOT in ChannelManager live-channel paths, so calling only monitor.signer_unblocked() (without node.signer_unblocked()) in enable_holder_signer_ops() is correct.
• disable_supported_ops_for_all_signers iterates over the same SUPPORTED_SIGNER_OPS array, so the disable/enable lifecycle is consistent.
• The finalization in settle_all correctly re-enables all ops and notifies both manager and monitor.
• The fuzz target binary needs no changes.

No issues found.