Re-enable signer op for one channel at a time in chanmon_consistency

[wpaulino]

Node B has two channels: one with A and another with C. Re-enabling signer ops only one channel at a time allows us to have them in different states, which may be helpful for fuzzing coverage.

Requested at #4411 (comment).

[ldk-reviews-bot]

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[ldk-claude-review-bot]

The split of node B's signer opcodes into per-channel variants correctly filters signer_unblocked, but enable_op_for_all_signers still enables the signer op globally for both of node B's channels. This means the two channels never actually have different signer states — only different notification states. See inline comment for details.