Keep directory sizing responsive and finalize recursive Size order

[frasergr]

Problem

Recursive directory sizes currently run inline on Flea's backend request loop. A large folder can therefore delay navigation, window refreshes, and clicks on another sort column for up to the walk's two-second deadline.

Size sorting also stops at a provisional order: files use exact metadata sizes, but folders remain in name order after their recursive sizes arrive.

Fix

• Run at most one recursive directory-size walk off the request loop, so list, sort, window, cancellation, and shutdown remain responsive.
• Check cancellation throughout traversal and preserve the existing two-second deadline and partial lower-bound behavior.
• Invalidate work by cache/row generation when a list or sort renumbers rows, preventing stale results from being published against reused indices.
• Keep completed values in a path-keyed cache and refresh visible directory cells after ordinary sorting.
• For Size sorting, queue each folder once, retain the immediate provisional response, then apply all recursive keys in one final reorder.
• Emit dirsorted after that reorder; the QML client clears row-indexed state, resets to row zero, requests the final window, and refills visible sizes from cache.
• Extend protocol coverage for cancellation, stale-result suppression, cache reuse, sort responsiveness, partial results, and deterministic final ordering.

Behavior preserved

• One directory walk at a time; no thread pool and no additional process.
• Symlinks are not followed.
• Viewport-only requests remain cancellable.
• Permission failures and timeouts still return an explicitly partial lower bound.
• Folder grouping, sort direction, and name tie-breaking remain deterministic.

Validation

• Backend protocol suite passed.
• QML/JavaScript checks: 3,101 passed.
• Ordering checks: 3 passed.
• Directory-size checks: 7 passed.
• Release build and file-budget gate passed.
• git diff --check passed.
• Full Rust run: 605 passed; one trash-recovery case failed in the full environment and passed when rerun in isolation.

Relationship to upstream PR thisisgm#118

Upstream PR #118 solves the same primary responsiveness bug with a smaller persistent-worker implementation. It covers off-loop traversal, cooperative cancellation, stale-result rejection, and shutdown behavior.

This branch adds behavior thisisgm#118 does not currently implement:

1. completed recursive sizes become the actual keys for a final Size reorder;
2. dirsorted tells the client that row indices changed;
3. QML clears stale row-indexed cells and fetches the final window;
4. directory sizes are refreshed correctly after other sorts.

If thisisgm#118 merges first, the minimal upstream path is to keep its worker and port only those exact-order/client-refresh pieces rather than merge both worker implementations.

Related but outside this PR: #144 is the same event-loop blocking class in synchronous MTP/GVFS scan(), not dirsize.