Native task-cache recording

Let the native backend produce task-cache entries, so "skip unchanged builds" works without the os sandbox — Linux hosts running the platform-branched native backend today get no task cache at all (persist falls back to a plain exec).

Why deferred: The replay half is already backend-agnostic: computeTaskCacheKey, resolveTaskCacheLocation, and replayTaskCache never touch an overlay — a native run could compute the key, hit, and apply the payload onto the host as-is. The blocker is recording: an entry needs a FlushOp[] plan describing what the command produced/deleted, and today that plan comes exclusively from probing the overlay upper (char-device whiteouts + user.overlay.* xattrs). Native runs in place with no upper, so there is no diff to probe — it would need a backend-agnostic change-detection mechanism (e.g. a pre/post working-tree diff via the same git plumbing the key hashing uses) feeding the same FlushOp[] shape, plus an in-process reconciler replacing the python-via-WSL transport. Doable, but a real seam to design — and the payoff is bounded: the dev-loop hosts that lean on the task cache today (win32) already run the os backend, and CI keeps the cache off by design.

Revisit when: A Linux-native dev loop (local Linux shells on this repo, or a consumer repo that pins native) demonstrably re-runs unchanged typecheck/lint/test often enough that replaying recorded runs would pay for the recording seam.