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]

👋 Thanks for assigning @tankyleo as a reviewer!
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Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.01%. Comparing base (647c3f7) to head (4f9b1e5).

Files with missing lines	Patch %	Lines
lightning/src/ln/channel_state.rs	66.66%	1 Missing ⚠️
lightning/src/ln/channelmanager.rs	0.00%	1 Missing ⚠️
lightning/src/routing/router.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4470      +/-   ##
==========================================
+ Coverage   85.98%   86.01%   +0.02%     
==========================================
  Files         159      159              
  Lines      105415   105424       +9     
  Branches   105415   105424       +9     
==========================================
+ Hits        90643    90679      +36     
+ Misses      12255    12235      -20     
+ Partials     2517     2510       -7     

Flag	Coverage Δ
tests	86.01% <66.66%> (+0.02%)	⬆️

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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.