Git checkout/branch operations now detect stale locks and layout changes

What

• The internal fast-forward helper used to advance a branch now takes commonDir and layout parameters, checks the branch ref lock, HEAD lock, and their .pid.lock companion files for staleness, adds a timeout to the underlying git calls, and re-verifies the on-disk layout hasn't changed before writing the new ref.
• The function that applies an incoming cloud-session commit to a local checkout now takes a layout parameter and, right before writing, re-checks that the checkout's git directory (its identity, HEAD, the session ref and reflog, packed-refs, and the shape of refs/ and objects/) hasn't changed since it was first read.
• Git working-tree status and conflict-state enums gain two new values: ref_locked and layout_changed, used to abort these operations instead of writing on top of a moved or altered checkout.

Why This protects against corrupting a repository when something else (another process, a concurrent session) has changed the git directory or holds a stale lock during a fast-forward or cloud-session sync, failing safely with a clear reason instead of writing bad state.