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!