Add Ctrl+click-drag to resize window splits

[mcrmck]

Summary

• Adds Ctrl+left-click-drag to resize window splits, a long-requested feature (see Using the mouse to resize splits? #5959)
• Detects which windows border the click position using neighbor info from the layout, then resizes in cell-sized increments during drag
• Shows a move cursor while dragging, restores default cursor on release
• Based on the patch by @pcewing from the discussion, with a fix for sparse NeighborsMap keys and the addition of a resize cursor

Changes

• kitty/mouse.c: Intercepts Ctrl+left-click to start drag resize, tracks mouse movement, ends on release. Sets move cursor during drag.
• kitty/boss.py: drag_resize_start/update/end methods that compute resize deltas in cell units and call resize_layout_window
• kitty/layout/base.py: drag_resize_target_windows method that determines which windows to resize based on click position and neighbor layout
• kitty/state.h: Adds active_drag_resize flag to GlobalState

Test plan

• All 162 existing tests pass
• Manual testing: horizontal resize with vertical border drag
• Manual testing: vertical resize with horizontal border drag
• Manual testing: mouse release stops resize
• Manual testing: normal click/select works without Ctrl held
• Manual testing: move cursor shown during drag, default cursor restored after

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