Function compiler_fingerprint 

pub fn compiler_fingerprint(
    binary_path: &Path,
    memo_dir: &Path,
) -> Result<CacheKey>

Compute or retrieve the cached hash of the compiler binary at binary_path. Memoization lives under memo_dir; callers typically pass ~/.cache/gruel/binary-hash.

On any I/O error reading the memo, falls back to recomputing. The cache is an optimization; correctness comes from the binary hash itself, not from the memo.