Add drag-and-drop quadrant preview overlay

During a window title-bar drag, render a semi-transparent tint over
the destination window to preview the drop outcome:

- Full-window tint (quadrant=5) when hovering any window in swap-based
  layouts (Tall, Stack, Fat, etc.) or over a title bar — swap is the
  result so the whole window is highlighted.
- Half-window directional tint (quadrant 1-4) when hovering in the
  Splits layout — a real directional insert happens so only the target
  half is highlighted.
- 150ms fade-in; clears immediately on drop or drag exit.

Implementation follows the visual_bell TINT_PROGRAM infrastructure:
two new Screen fields (start_drag_overlay_at, drag_overlay_quadrant),
draw_drag_preview_overlay() in shaders.c called from draw_cells() after
both render paths, set_window_drag_overlay() C→Python bridge in state.c,
and the same child-monitor.c needs_render + set_maximum_wait hooks that
visual_bell uses to keep frames firing during animation.

Layout detection uses hasattr(layout, 'insert_window_next_to') — the
same guard _insert_window_in_direction uses internally — so the overlay
always matches the actual drop behavior.

kitty/screen.h

@@ -129,6 +129,8 @@ typedef struct {
     ScreenModes modes, saved_modes;
     ColorProfile *color_profile;
     monotonic_t start_visual_bell_at;
+    monotonic_t start_drag_overlay_at;  // 0 = inactive
+    uint8_t drag_overlay_quadrant;      // 1=left 2=right 3=top 4=bottom 0=none
 
     uint8_t *write_buf;
     size_t write_buf_sz, write_buf_used;