Complete the Focus Manager (mouse-follows-focus, focus-follows-mouse, monocle/stack restacking)

## Problem / goal

Complete the Focus Manager: synchronous user-driven focus changes, mouse-follows-focus, OS-delegated focus-follows-mouse, and monocle/stack restacking.

## References

- `DESIGN.md` §7 (Focus paragraph, in full) — synchronous-from-user-input-only focus changes via `SetForegroundWindow` + `GetForegroundWindow` verification + `AllowSetForegroundWindow` retry; mouse-follows-focus via `SetCursorPos` to frame-bounds center on `EVENT_SYSTEM_FOREGROUND` with self-caused-change suppression; focus-follows-mouse delegated to the OS via `SPI_SETACTIVEWINDOWTRACKING` (+TRKZORDER/TRKTIMEOUT), original SPI values captured on start and restored on exit; never `AttachThreadInput` or synthetic-Alt tricks; monocle/stack restacking via per-window `SetWindowPos(hwndInsertAfter, SWP_NOMOVE|SWP_NOSIZE|SWP_NOACTIVATE)`.

## Acceptance criteria

- [ ] Bastion changes focus only synchronously from the user's own input, verifying via `GetForegroundWindow` with one `AllowSetForegroundWindow` retry, accepting the OS's taskbar-flash degradation otherwise.
- [ ] Mouse-follows-focus moves the cursor to the newly-focused window's frame-bounds center on `EVENT_SYSTEM_FOREGROUND`, suppressing self-caused re-triggering.
- [ ] Focus-follows-mouse is implemented purely via `SPI_SETACTIVEWINDOWTRACKING` (+ TRKZORDER/TRKTIMEOUT) — original SPI values are captured at startup and restored on exit; `AttachThreadInput`/synthetic-Alt tricks are never used.
- [ ] Monocle/stack restacking uses `SetWindowPos(hwndInsertAfter, SWP_NOMOVE|SWP_NOSIZE|SWP_NOACTIVATE)`, honoring documented z-order/owned-window rules.

Start with DESIGN.md §7, especially the complete Focus paragraph, and map its requirements to the Focus Manager entry points. Verify synchronous foreground changes, OS focus tracking, cursor-move suppression, and monocle/stack restacking against the documented Windows API behavior. Done means all four acceptance-criteria groups pass without AttachThreadInput or synthetic-Alt techniques, with original SPI values restored on exit.

Written by the indexing model from the issue text.