Replace hashes_fuzz with local fuzz gates

[joostjager]

Removes the global hashes_fuzz mode and replaces it with small local cfg(fuzzing) exceptions at the specific hash/MAC verification gates that matter for fuzz coverage.

This keeps hashing real by default, updates the few harnesses that depended on fake hashes, and removes the extra hashes_fuzz target/config/CI plumbing. A quick profiling session showed that for chanmon_consistency, real hash usage increases runtime by about 15%, which seems acceptable.

We don’t toggle hashes_fuzz per target because it is a build-time cfg on the whole fuzz crate. Doing that cleanly would require target-specific generator/build logic or splitting targets into separate packages, while local gate exceptions give us the same coverage benefit with much less complexity and a smaller blast radius.

[ldk-reviews-bot]

👋 Hi! I see this is a draft PR.
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

❌ Patch coverage is 91.11111% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.85%. Comparing base (5704e8e) to head (b386316).
⚠️ Report is 21 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-liquidity/src/lsps2/utils.rs	66.66%	3 Missing ⚠️
lightning/src/util/fuzz_wrappers.rs	94.44%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4551      +/-   ##
==========================================
- Coverage   86.99%   86.85%   -0.15%     
==========================================
  Files         163      163              
  Lines      108635   108703      +68     
  Branches   108635   108703      +68     
==========================================
- Hits        94511    94417      -94     
- Misses      11647    11778     +131     
- Partials     2477     2508      +31     

Flag	Coverage Δ
fuzzing	39.28% <37.20%> (-1.03%)	⬇️
tests	86.07% <95.12%> (-0.03%)	⬇️

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