perf: resolve stale-file last-touch with one git walk

[stephenc222]

Summary

• Implements hotspots-research/docs/promotion-briefs/last-touch-single-walk.md.
• git.rs::batch_last_touch_for_files resolved days_since_last_change for files with no commit in the 30-day window by running one full-history, path-limited git log -- <500 paths> per 500 files. Each call walks the whole history, so --touch-mode file cost grew with history length times stale files: 232s on vscode, of which the fallback is essentially all (the 30-day window query itself takes 0.01s).
• Replaced with one unrestricted git log --format="COMMIT %ct" --name-only --until=<as_of> walk. Signature and value semantics unchanged.
• Measured (research POC): vscode 397.6s -> 6.1s (~65x), scikit-learn 14.4s -> 0.8s (18x) for the fallback alone.

Behavior note (intended)

The old path-limited walk applies git history simplification, which can prune a side-branch commit at a merge that is TREESAME to the first parent. The single walk sees those commits, so a value is never older than before and is occasionally more recent. In the vscode POC 28 of 14,247 stale files differed (0.2%): 23 strictly more recent, 5 within a day, 0 older. On linear history the results are identical. A fixture test documents the difference.

Test plan

• Linear-history fixture: new map equals a test-only copy of the old chunked implementation exactly, with expected day values asserted
• Merge fixture (-s ours): new never older than old; f.txt old 10 days, new 8
• Commits after as_of ignored; files absent from history omitted; empty input spawns no git
• cargo test --workspace (555 in hotspots-core), cargo fmt --check, cargo clippy --workspace --all-targets clean
• Old (v1.40.0) vs new binary on real repos, --touch-mode file: days_since_last_change identical for every function on this repo (982), scikit-learn (3,826) and vite (2,102), none larger; wall time 2.2s->1.6s, 9.3s->4.9s, 2.2s->1.0s

Not in scope

• git quotes non-ASCII paths (core.quotePath), so such files never match stale_files in either method. Noted in the brief as a separate change.

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