Function grouping_cache_dir 

pub fn grouping_cache_dir(common_dir: &Path) -> PathBuf

The grouping cache directory for a repo, given its git common dir.

Under the common dir, not the worktree’s .git, so worktrees of one repo share a cache — and outside the tracked tree, since a grouping is a local artefact and not something to commit (ADR 0009).