Equalize tile sizes after every BSP tree operation

After each drag-drop, add, or remove, walk the tree and reset each split's
ratio to left_leaves/(left_leaves+right_leaves). This distributes space
equally per pane regardless of the prior tree structure.

- _tree_equalize_ratios: recomputes all ratios bottom-up by leaf count
- _tree_leaf_count: counts leaves in a subtree
- _tree_grid_size: uses LCM of split denominators (capped at 12) instead
  of 2^depth, giving exact integer rowspans for equal distributions
- _tree_default: balanced binary tree + equalize (not uneven chain)
- Applied in _do_drop, _add_pane, _rm

Result: 3 side-by-side panes → equal widths; any mixed layout → equal area.

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

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