Tiled Model layouts now show the inner edges between panes as 2-px
dividers that act as resize grips: cursor on a divider → resize cue,
press + drag moves the edge (with every tile sharing that edge
updating together), release runs a collapse pass that absorbs any
tile that fell below the viewcube-comfort minimum into the neighbour
with the longest shared contact edge — drag a divider all the way
across to remove a pane.
- Scene grows `model_tile_edges`, `hit_model_tile_edge`,
`move_model_tile_edge`, `collapse_small_model_tiles`. Edges are
derived from the existing `model_tiles` rects (no extra storage).
- App carries a `tile_drag` Option; `ViewportLeftPress` starts it
ahead of tile activation / picking, `ViewportMove` drives the
edge, `ViewportLeftRelease` collapses + clears.
- `SelectionCanvas` draws the dividers and switches the cursor to
`Resizing{Horizontally, Vertically}` over them.
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