Function repo_cache_dir 

pub fn repo_cache_dir(cache_root: &Path, repo_path: &Path) -> PathBuf

Compute the per-repo cache subdirectory: <cache_root>/codelore/<repo_hash_8>/

All per-repo sidecar files (gate ledger, external findings) share this directory with the .duckdb cache entries. The 8-hex-char directory name is sha256(canonical_repo_path)[0..4] encoded as lowercase hex.

This is the canonical implementation — cache_path_with_root and quality_gates::ledger::ledger_dir both delegate here so the derivation is defined in exactly one place.