fuzz: improve log performance

[joostjager]

It turned out that null logging during fuzzing was still a break on performance because of unconditional format!'ing. 2.5x speed up in CI.

[ldk-reviews-bot]

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[ldk-claude-review-bot]

No issues found.

Details:

• fuzz/src/utils/test_logger.rs (TypeId optimization): The TypeId::of::<Out>() == TypeId::of::<DevNull>() check is correct. Since TestLogger<DevNull> is monomorphized at compile time, LLVM will constant-fold this comparison to true and eliminate the function body, making log() a no-op. Even when called through dynamic dispatch (Arc<dyn Logger>), the monomorphized implementation retains the optimization. The Output trait already has the required 'static bound for TypeId.

• fuzz/src/chanmon_consistency.rs (SearchingOutput removal): The searched string "Outbound update_fee HTLC buffer overflow - counterparty should force-close this channel" no longer exists anywhere in the codebase, confirmed by grep. This means may_fail was always false, so all removed conditional branches were already unconditional panics. The removal is a correct dead-code cleanup with no behavior change.

• Parameter rename (underlying_out -> out): Correct — SearchingOutput::new(underlying_out) previously produced out; now the parameter is used directly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.21%. Comparing base (688544d) to head (d6ff54e).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4523      +/-   ##
==========================================
+ Coverage   86.14%   86.21%   +0.06%     
==========================================
  Files         160      160              
  Lines      108046   108410     +364     
  Branches   108046   108410     +364     
==========================================
+ Hits        93080    93461     +381     
+ Misses      12346    12318      -28     
- Partials     2620     2631      +11     

Flag	Coverage Δ
tests	86.21% <ø> (+0.06%)	⬆️

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[TheBlueMatt]

Hmm, we do want the ability to catch debug asserts in Display/Debug impls. Where is the perf improvement - is it focused on individual fuzzers?

[joostjager]

Hmm, we do want the ability to catch debug asserts in Display/Debug impls. Where is the perf improvement - is it focused on individual fuzzers?

For the fuzz CI job for chanmon consistency, it goes down from 10 to 4 minutes. There is just a lot being formatted, but then not actually logged. For catching asserts in display/debug, fuzzing seems not the most efficient way to do it.