Tidying around intrinsics, safety comments, and pointer casts

[ctz]

This is a reduced-strength version of #107. #107 introduces some performance changes that I haven't investigated yet.

[Copilot]

Pull Request Overview

This PR modernizes pointer casting in cryptographic code by replacing explicit cast-as-pointer patterns with .cast() method calls, consolidates duplicate prefetching logic into a reusable function, and adds safety comments for memory transmutations.

• Replaces as *const _ and as *mut _ pointer casts with .cast() method calls
• Consolidates duplicate prefetching code into a shared super::cpu::prefetch() function
• Adds safety comments for mem::transmute operations and improves existing safety documentation

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
graviola/src/lib.rs	Adds clippy lint exceptions for pointer casting patterns
graviola/src/low/x86_64/cpu.rs	Introduces shared prefetch function for code deduplication
graviola/src/low/x86_64/sha256.rs	Updates pointer casts to use .cast() method
graviola/src/low/x86_64/ghash.rs	Updates pointer casts to use .cast() method
graviola/src/low/x86_64/aes_gcm.rs	Replaces duplicate prefetch code with shared function and updates pointer casts
graviola/src/low/x86_64/aes.rs	Updates pointer casts to use .cast() method
graviola/src/low/x86_64/bignum_*.rs	Replaces duplicate prefetch code with shared function
graviola/src/low/aarch64/*.rs	Updates pointer casts and adds safety comments for transmute operations
graviola/src/low/generic/zeroise.rs	Updates pointer casts to use .cast() method

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.14%. Comparing base (a177593) to head (1c46112).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #108      +/-   ##
==========================================
- Coverage   99.14%   99.14%   -0.01%     
==========================================
  Files         171      171              
  Lines       39090    39074      -16     
==========================================
- Hits        38754    38738      -16     
  Misses        336      336              

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[codspeed-hq]

CodSpeed Performance Report

Merging #108 will not alter performance

_{Comparing jbp-safer-intrinsics-2 (1c46112) with main (a177593)}

Summary

✅ 143 untouched benchmarks