Replace grid-snap canvas with BSP-tree drag-to-split tiling

Dragging a tile to the edge (left/right/top/bottom) of another tile
splits that tile's space and inserts the dragged tile on that side.
Drop zones occupy the outer third of each tile edge; center has no zone.
Ghost overlay shows the split target during drag.

Data model: LayoutConfig.tree (BSP tree as nested dict). PaneSpec.row/col
removed. strip_chart uses tree_to_grid() → rowspan/colspan for pyqtgraph.
Backward compat: tree=None falls back to layout_mode stacking path.

Example: 3 stacked → drag Tile 3 to left of Tile 1 → Tile 3|Tile 1 on
top (side-by-side), Tile 2 spans full bottom.

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

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