Implement drag-to-swap with a click-through preview overlay

## Problem / goal

Implement drag-to-swap: dragging a tiled window over another shows a preview of the resulting swap and re-tiles once the drag ends.

## References

- `DESIGN.md` §7 (Input & Focus, Drag-to-swap paragraph) — `EVENT_SYSTEM_MOVESIZESTART`/`END` bracket the drag; a WM-owned click-through highlight window (`WS_EX_LAYERED|WS_EX_TRANSPARENT|WS_EX_NOACTIVATE` + `SetLayeredWindowAttributes`) previews the drop tile; re-tile happens once on MOVESIZEEND via a Defer batch.
- Depends on the WinEvent ingest pump issue (source of MOVESIZESTART/END) and the Placement Executor issue (the Defer batch mechanism reused here).

## Acceptance criteria

- [ ] A drag gesture is bracketed by `EVENT_SYSTEM_MOVESIZESTART`/`END` (consumed from the existing WinEvent ingest pump, not a second hook).
- [ ] A layered, click-through, non-activating overlay window previews the candidate drop tile during the drag.
- [ ] Exactly one re-tile Defer batch is issued on `MOVESIZEEND`, not continuously during the drag.

Start with DESIGN.md §7, then read the existing WinEvent ingest pump and Placement Executor work referenced by the issue. Trace how MOVESIZESTART/END are consumed and how the Defer batch is reused. Done means a click-through preview appears during the drag and exactly one re-tile batch runs on MOVESIZEEND.

Written by the indexing model from the issue text.