Fix split zone detection and canvas-edge squeeze behavior

Split zone was unreachable: squeezed between outer squeeze (1/6) and
center (1/3), leaving a 1/6-wide band impossible to target reliably.
Users hitting the tile edge always triggered squeeze instead of split.

- Remove squeeze from inside-tile detection entirely. Split now owns
  the outer 1/3 of each tile edge — large, easy to target. Center
  keeps the inner 1/3. Squeeze remains gap-detection only (second pass).

- Fix canvas-edge squeeze (no adjacent neighbor): instead of bisecting
  the edge tile, wrap the entire remaining tree in a new directional
  split with the dragged tile placed at the outer position, then
  equalize — all tiles end up equal size.

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

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