Three-zone drop system: squeeze, split, center

Zone layout (outer to inner per edge):
  0–1/6    squeeze (amber): insert dropped tile between two existing tiles
  1/6–1/3  split (blue):    bisect the target tile 50/50
  1/3–2/3  center (green):  swap/move

Squeeze behaviour: find the tile adjacent to the target in the drop
direction; insert dragged tile to the opposite side of that neighbor
so it lands visually between them. Falls back to split when no neighbor
exists (edge tile).

- _zone_at: returns "squeeze_*"/"split_*"/"center" with dominant-axis
  selection (nearest edge wins)
- _adjacent_tile: finds geometrically adjacent tile by pixel proximity
- _do_drop: routes squeeze→insert-at-neighbor, split→bisect, center→swap
- paintEvent: amber/blue/green per zone type

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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