fuzz: Add upgrade/downgrade simulation to chanmon_consistency and fix chacha20 build

[Atishyy27]

Addresses #4452

This PR introduces real cross-version serialization coverage to the chanmon_consistency fuzzer by depending on a previous version of LDK (lightning_0_2).

Changes:

• Test Harnesses: Updated FuzzEstimator, TestBroadcaster, and TestChainMonitor to implement both the current traits and the lightning_0_2 versions of those traits.
• Refactoring: Abstracted common logic (like monitor byte persistence) into private helper methods to avoid code duplication between trait versions.
• Dependencies: Added lightning_0_2 (v0.2.0) as a dependency in fuzz/Cargo.toml.
• Fuzzer Logic: Updated the reload_node closure to support a cross-version serialization round-trip path.

[ldk-reviews-bot]

👋 Hi! This PR is now in draft status.
I'll wait to assign reviewers until you mark it as ready for review.
Just convert it out of draft status when you're ready for review!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.44%. Comparing base (cb951b4) to head (2358ca7).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4499      +/-   ##
==========================================
+ Coverage   86.18%   87.44%   +1.26%     
==========================================
  Files         160      160              
  Lines      107441   114503    +7062     
  Branches   107441   114503    +7062     
==========================================
+ Hits        92593   100125    +7532     
+ Misses      12229    11755     -474     
- Partials     2619     2623       +4     

Flag	Coverage Δ
tests	87.44% <ø> (+1.26%)	⬆️

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[TheBlueMatt]

Huh? In order to do an upgrade/downgrade we'll need to actually depend on a previous version of LDK.

[Atishyy27]

Ah, I see! I was initially aiming for forward-compatibility coverage by simulating unknown odd TLVs.

If the goal is actual cross-version testing, should I add a previous version (e.g., 0.0.124 or similar) as a renamed dev-dependency in fuzz/Cargo.toml to perform real comparisons? I'd appreciate a pointer if there's an existing target using this pattern I should follow.

[TheBlueMatt]

See the upgrade tests in tests. Probably just start with 0.2.

[Atishyy27]

Got it. I checked upgrade_downgrade_tests.rs and see the pattern of importing lightning_0_2 alongside the current crate.

Before I go too deep into the implementation for the fuzzer: since lightning_0_2's traits (like chain::Watch, FeeEstimator, Router, etc.) are entirely distinct from the current lightning crate's traits, doing a full read/reload cycle with the older version inside chanmon_consistency.rs would require duplicating a lot of the fuzzer's test harness structs (e.g., TestChainMonitor, FuzzEstimator, KeyProvider) to implement the 0_2 traits.

Shall I go ahead and duplicate those test harness implementations for the 0.2 dependency, or is there a lighter-weight serialization round-trip pattern you had in mind for the fuzzer?

[TheBlueMatt]

I was hoping we'd be able to keep the existing test harnesses but just implement both traits on them - moving common logic to util methods to avoid duplicate code.

[ldk-claude-review-bot]