Fix branch snapshot isolation

[congwang-mk]

Summary

• Each branch now captures a frozen snapshot of its parent's visible tree (delta + inherited view) at fork time into a per-branch inherited/ directory, instead of walking the live ancestor chain on every lookup.
• resolve_path and collect_dir_names now consult only the branch's own delta plus its frozen inherited snapshot; the live base is only used by main. This makes a branch's view immune to subsequent writes on its parent.
• Path resolution is refactored into _locked helpers so the snapshot can be built atomically while holding the branches lock; failure during snapshotting cleans up the partially-created branch directory.

Test plan

• cargo test
• tests/test_branch_dirs.sh
• tests/test_commit.sh
• Manual: create a branch, mutate parent, confirm branch's view is unchanged

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