Derive per-channel destination and shutdown scripts for improved on-chain privacy

[Abeeujah]

Generate unique per-channel destination and shutdown scripts when V2 remote key derivation is enabled. Previously,
KeysManager used a single global destination_script and shutdown_pubkey for all channels, causing address reuse
across channels.

Changes:

1. 04386c1 Derive per-channel destination/shutdown scripts from channel_keys_id.
2. d429646 get_shutdown_script_pubkey now takes channel_keys_id to produce unique cooperative-close scripts per channel.
3. 82e54e3 Add comprehensive tests verifying scripts are per-channel, distinct, non-legacy, and recoverable.
4. e458aad Add changelog entry.

Funds sent to per-channel scripts remain recoverable from seed alone via possible_v2_static_output_spks. A new
find_static_output_key method supports spending both legacy (global) and per-channel keys.

closes #1139

[ldk-reviews-bot]

I've assigned @wpaulino 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]

No new issues found.

This is the same commit (0170743…) as my previous review pass, and I re-verified the new logic in depth:

• Backwards compatibility is correct: destination_key/shutdown_key are derived at m/1' and m/2' (DESTINATION_SCRIPT_INDEX/SHUTDOWN_PUBKEY_INDEX), the same indices the old spend_spendable_outputs path used via a fresh Xpriv::new_master(Network::Testnet, seed), so previously-created StaticOutputs remain spendable after upgrade.
• find_static_output_key checks legacy roots first, then per-channel children — covers v1 channels, v2 channels (with channel_keys_id), and seed-only recovery (full scan).
• static_output_key_idx distinct-index guarantee holds: counter is channel_keys_id[0..4] (incrementing) and the offset hashes [4..16] (constant starting-time bytes within an instance).
• create_chanmon_cfgs_with_legacy_keys passes predefined_keys_ids.is_some() into the v1_derivation parameter — correct (Some ⇒ legacy V1).
• use bitcoin::hex::FromHex in functional_tests.rs is the only such import (no duplicate).

Correction to my prior review: my earlier "compile failure" claim about fuzz/src/onion_message.rs:312 was a false positive — that file is included in this diff and is correctly updated to the new (&self, _channel_keys_id: [u8; 32]) signature. All SignerProvider implementors are updated.

The previously-posted inline comments still stand (they are unchanged in this commit):

• lightning/src/sign/mod.rs:2027 — broken intra-doc link (space instead of _).
• lightning/src/sign/mod.rs:2607 / :2423 — comment typos for possible_v2_static_output_spks.
• pending_changelog/1139.txt:2 — wrong method name get_shutdown_script_pubkey.

Non-blocking note (carried over): for externally-supplied channel_keys_ids the per-channel index can collide, so script reuse is possible — acceptable per the documented behavior.

[codecov]

Codecov Report

❌ Patch coverage is 96.23656% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.97%. Comparing base (43ea836) to head (0170743).

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	70.00%	3 Missing ⚠️
lightning/src/sign/mod.rs	98.24%	2 Missing and 1 partial ⚠️
lightning/src/util/test_utils.rs	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #4756    +/-   ##
========================================
  Coverage   86.96%   86.97%            
========================================
  Files         161      161            
  Lines      111648   111784   +136     
  Branches   111648   111784   +136     
========================================
+ Hits        97099    97224   +125     
- Misses      12045    12053     +8     
- Partials     2504     2507     +3     

Flag	Coverage Δ
fuzzing-fake-hashes	8.43% <14.03%> (-0.01%)	⬇️
fuzzing-real-hashes	32.38% <3.50%> (-0.04%)	⬇️
tests	86.30% <96.23%> (+<0.01%)	⬆️

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