Largest single clone-performance win available. .bitmap files store reachability bitmaps per commit, letting the packfile writer skip the reachability walk on clone.

Scope

• Reader: load .bitmap alongside a pack, expose reachable_from(commit_sha).
• Writer: produce .bitmap when writing a pack with --bitmap flag.
• Integration: UploadPack.Writer uses bitmaps when present to pick objects for the output pack without walking the DAG.

Acceptance

• A 100k-commit fixture clone goes from tens of seconds to under 1s with bitmaps vs without.
• Interop: git client accepts the bitmap-generated pack; git verify-pack is happy.
• Correctness: same object set as the naive walker for arbitrary want/have pairs.