Function git_init_boundary 

pub fn git_init_boundary(dir: &Path) -> bool

v0.8: make dir a standalone git repository (best-effort) so discover_repo_root resolves to dir itself instead of climbing to an enclosing repo. Two callers:

• the benchmark harness, for throwaway fixture repos — without this, a fixture created under the system temp dir on a machine whose $HOME (or any ancestor) is a git repo would init its brain at that ancestor and leak fixture memories into it;
• tests that create isolated project roots under the temp dir.

Creates dir if needed. Returns true when git reported success; a failure (e.g. git not installed) just means the caller doesn’t get isolation, which is the prior behaviour.