Avoid splice checks when responding to stfu

[wpaulino]

Only gate local quiescence initiation on splice RBF eligibility. If the counterparty initiated quiescence first, respond with non-initiator stfu once pending channel updates are clear.

[ldk-reviews-bot]

👋 Thanks for assigning @jkczyz 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 now thoroughly reviewed every hunk in this diff. Let me trace through the key scenario to verify correctness:

Scenario: Node 0 has a pending quiescent_action (Splice at floor feerate) but can't initiate because RBF feerate is too low. Node 1 sends stfu.

Old code path (buggy):

1. stfu() handler sets REMOTE_STFU_SENT, calls try_send_stfu()
2. try_send_stfu() checks quiescent_action → finds Splice → RBF feerate check fails → returns None
3. Node 0 never responds to Node 1's stfu — stuck

New code path (fixed):

1. stfu() handler sets REMOTE_STFU_SENT, calls try_send_stfu()
2. try_send_stfu() checks pending updates (passes), then checks is_remote_stfu_sent() → true → enters non-initiator path
3. Clears REMOTE_STFU_SENT, sets QUIESCENT, returns Stfu { initiator: false }
4. quiescent_action stays pending for retry after quiescence ends

The fix is correct. The guard at line 14624 guarantees the else branch is unreachable (if is_remote_stfu_sent() is false, then quiescent_action.is_some() must be true). The test exercises exactly this scenario.

No issues found.

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.63%. Comparing base (ee456a8) to head (dbb1c96).

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	80.00%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #4630       +/-   ##
===========================================
+ Coverage   28.02%   86.63%   +58.61%     
===========================================
  Files         126      159       +33     
  Lines       69960   110500    +40540     
  Branches    69960   110500    +40540     
===========================================
+ Hits        19606    95735    +76129     
+ Misses      49020    12230    -36790     
- Partials     1334     2535     +1201     

Flag	Coverage Δ
fuzzing-fake-hashes	6.62% <0.00%> (-0.01%)	⬇️
fuzzing-real-hashes	23.25% <0.00%> (-0.01%)	⬇️
tests	86.24% <80.00%> (?)

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[jkczyz]

IIUC, we would attempt to contribute as an acceptor, assuming we are both initiating for a splice.

[ldk-reviews-bot]

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