Add multi-window arrangers (grid / cascade) over the layout planner

[JE-Chen]

Lay out a whole set of windows in one call — snap_window moves one window and the v133 layout planner only computes rectangles; these close the loop.

• arrange_grid (AC_arrange_grid): tile a list of window titles into an R×C grid (defaults to a near-square auto-shape for the window count), with an optional gap. Windows beyond the grid capacity are left untouched.
• arrange_cascade (AC_arrange_cascade): stagger the titles diagonally, each offset px down-right of the previous, sized to 60% of the work area.
• Build on the layout planner for geometry and reuse snap_window's injectable mover / screen_size seams, so the arrangement logic is fully headless-testable (the tests inject a recording mover); both return the count moved. Default mover is Win32 today, other platforms no-op until their backend lands.
• Wired through all 5 layers (core in window_capture / facade / executor / MCP [SIDE_EFFECT_ONLY] / Script Builder under Window) + headless test + EN/Zh docs + WHATS_NEW. Qt-free.

[codacy-production]

Not up to standards ⛔

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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