chancloser: account for aux close outputs in initial coop close fee baseline

[darioAnongba]

Reopens #10615, which was auto-closed by GitHub when my fork was deleted. Same commit, same diff.

Problem

Custom channel coop closes can fail with: unable to process close msg: insufficient fee.

Root cause

For taproot/custom channels, ChanCloser computed the initial coop close fee (idealFeeSat) from only local+remote outputs, but the final close tx can include auxiliary extra outputs.

That mismatch can underprice the first accepted close offer (notably in taproot fast-close), causing mempool rejection and peer disconnect.

Fix

Extend coop close fee weight calculation to include optional extra outputs.

[github-actions]

🔴 PR Severity: CRITICAL

gh pr view | 3 files | 206 lines changed

🔴 Critical (1 file)

• lnwallet/chancloser/chancloser.go - lnwallet/* package; implements the cooperative channel-closing state machine (fee negotiation, closing-signature exchange), which directly affects channel funds settlement.

🟢 Low (2 files)

• lnwallet/chancloser/chancloser_test.go - test-only change, excluded from severity-bump counting but reviewed alongside the production code.
• docs/release-notes/release-notes-0.21.0.md - release notes update.

Analysis

The core production change is in lnwallet/chancloser/chancloser.go, which sits under lnwallet/* — explicitly listed as a CRITICAL package because it governs channel funding/closing and commitment-related logic. Since chancloser is responsible for negotiating and finalizing on-chain closing transactions, correctness here has direct financial-safety implications, warranting expert review regardless of the modest diff size (86 additions / 11 deletions in the file, well under the >20 files / >500 lines bump thresholds — no severity bump was needed since CRITICAL is already the ceiling).

No override labels were present, so this classification is based purely on file-path rules.

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