fuzz: add chanmon stuck HTLC invariant

[joostjager]

This adds a chanmon consistency fuzz invariant that checks for stuck channel HTLCs after the harness has settled all state. The previous corpus signal was indirect, usually showing up as a later capacity assertion failure, so the new invariant makes the oracle point at the actual leftover HTLC state.

The invariant reproduced the stuck-HTLC issue on upstream/main with this corpus entry:

• 0270801109191109191f1f10b6ff

Verified that #4520 fixes the issue.

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt 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]

The diff is a single, well-placed assertion block. My prior review covered all the substantive analysis. There are no new issues to flag.

No issues found.

This is a clean, well-placed invariant check in settle_all() that asserts no HTLCs remain pending after the harness has fully settled state. The prior review already confirmed correctness of the Debug formatting, channel iteration scope, placement within the settlement sequence, and diagnostic info in the assertion message. No new issues identified on re-review.

[joostjager]

Merge order: #4571 first if no new changes needed there.

[joostjager]

Rebased after merge of #4571