feat: allow configurable force-close buffer for claimable HTLCs

[okekefrancis112]

This PR adds a configurable force_close_claimable_htlc_cltv_buffer to ChannelConfig, allowing users to control how far in advance an inbound channel is force-closed when an HTLC becomes claimable. Previously, this buffer was hardcoded to CLTV_CLAIM_BUFFER (36 blocks), limiting users’ ability to tolerate their own downtime. The new field defaults to the existing constant, enforces the same minimum, and is validated against cltv_expiry_delta to preserve safety guarantees. The value is propagated to ChannelMonitor, persisted via monitor updates and serialization, and used in place of the hardcoded constant when deciding whether to broadcast a holder commitment transaction. This change improves configurability without altering default behavior.

Closes: #3837

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[codecov]

Codecov Report

❌ Patch coverage is 67.28972% with 35 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.04%. Comparing base (2e4a2ac) to head (711eadc).
⚠️ Report is 32 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/chain/channelmonitor.rs	53.65%	18 Missing and 1 partial ⚠️
lightning/src/chain/chainmonitor.rs	66.66%	10 Missing ⚠️
lightning/src/ln/channelmanager.rs	53.84%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4401      +/-   ##
==========================================
+ Coverage   86.02%   86.04%   +0.02%     
==========================================
  Files         156      156              
  Lines      103100   103477     +377     
  Branches   103100   103477     +377     
==========================================
+ Hits        88693    89040     +347     
- Misses      11896    11925      +29     
- Partials     2511     2512       +1     

Flag	Coverage Δ
tests	86.04% <67.28%> (+0.02%)	⬆️

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[TheBlueMatt]

There's a bunch of checks in channelmanager.rs that need to be equivalently enforce on settings. In any case, please carefully go through every use of CLTV_CLAIM_BUFFER and maybe all the block-count-constants read the associated comments, and consider if we should update anything or if it might imply changes somewhere.

[TheBlueMatt]

Bleh, pushing through a monitor update is a heavy persistence operation. Instead, let's just make this configurable on the ChainMonitor and keep the settings memory-only.

[okekefrancis112]

Thank you for pointing this out. Your review is greatly appreciated, I will work on this and push it.