Expose current dust exposure in ChannelDetails

[Bortlesboat]

Summary

Adds a current_dust_exposure_msat field to ChannelDetails that surfaces the current total dust exposure on a channel.

• The value is the maximum of the dust exposure across the holder and counterparty commitment transactions
• Includes both dust HTLC values and the commitment transaction fee component (as noted by @TheBlueMatt in Expose current dust exposure #2264, dust exposure now includes commitment tx fees, not just dust HTLCs)
• Users can compare this against ChannelConfig::max_dust_htlc_exposure to monitor how close a channel is to its dust limit
• Field is Option<u64>, None for objects serialized prior to 0.2.1
• TLV tag 49 for backwards-compatible serialization

Implementation

1. Added dust_exposure_msat to AvailableBalances — computed as max(local_dust_exposure_msat, remote_dust_exposure_msat) in get_available_balances (tx_builder.rs)
2. When folding across multiple funding scopes (splices), takes the max dust exposure
3. Added current_dust_exposure_msat: Option<u64> to ChannelDetails struct, populated from the balance computation in from_channel

Fixes #2264

[ldk-reviews-bot]

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[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.19%. Comparing base (4a3aaef) to head (515b244).
⚠️ Report is 97 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel_state.rs	66.66%	1 Missing ⚠️
lightning/src/routing/router.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4470      +/-   ##
==========================================
+ Coverage   86.01%   86.19%   +0.17%     
==========================================
  Files         159      160       +1     
  Lines      105430   107554    +2124     
  Branches   105430   107554    +2124     
==========================================
+ Hits        90690    92707    +2017     
+ Misses      12229    12221       -8     
- Partials     2511     2626     +115     

Flag	Coverage Δ
tests	86.19% <75.00%> (+0.17%)	⬆️

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[ldk-reviews-bot]

🔔 1st Reminder

Hey @tankyleo! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

[Bortlesboat]

Quick follow-up from my side: I’m treating review-required PRs as top priority this week. If you want any specific changes, rebase, or split, I can turn them around quickly.

[ldk-reviews-bot]

🔔 2nd Reminder

Hey @tankyleo! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

[ldk-claude-review-bot]

I've reviewed the entire PR diff thoroughly, checking all changed files against the codebase context I have from prior reviews.

Prior review status:

• The #[allow(deprecated)] issue (tx_builder.rs) — fully resolved, the annotation was removed.
• The version string issue (channel_state.rs:493) — partially resolved. It was changed from "prior to 0.2.1" to "prior to 0.3." but the suggestion was "prior to 0.3.0" to match the three-part version convention used elsewhere (e.g., "prior to 0.2.0" on line 480). This is a minor inconsistency, not a bug.

New issues: None found. The implementation is correct:

• max(local_dust_exposure_msat, remote_dust_exposure_msat) correctly captures worst-case dust exposure.
• The fold across funding scopes uses .max() — correct for worst-case across splices.
• TLV tag 49 (odd, sequential after 47) is correct for backwards-compatible optional serialization.
• All 4 AvailableBalances construction sites and all ChannelDetails construction sites include the new field.
• Serialization round-trip test at line 777 covers the new field.
• Option<u64> in ChannelDetails (with None for old serialized data) vs plain u64 in AvailableBalances is appropriate.
• Error/fallback paths default dust_exposure_msat to 0, consistent with the existing pattern for other balance fields in those paths.

No issues found.

[ldk-reviews-bot]

🔔 3rd Reminder

Hey @tankyleo! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

[Bortlesboat]

Hi @tankyleo — no worries if you're busy! Could another maintainer pick this up? Happy to address any feedback quickly.

[ldk-reviews-bot]

🔔 4th Reminder

Hey @tankyleo! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.

[tankyleo]

Sorry for the delay

[Bortlesboat]

Hi — tankyleo appears to be unavailable (4 reminders sent). Could another reviewer pick this up? It's a small addition exposing dust exposure values that are already computed internally. Happy to address any feedback.

[TheBlueMatt]

Two small things, one that actually needs fixing, otherwise we'll land this. Thanks!

[TheBlueMatt]

Please feel free to squash the fixup commits down so that the git history only contains the one main commit

[Bortlesboat]

Done — squashed into a single commit.

[TheBlueMatt]

Please address the claude comment - its correct, we should drop that allow i think it snuck in from a back rebase. Also please feel free to squash the fixup commit again.