mouseSelectionButton: 2 breaks panning and selection

Describe the bug
Since v5.11.2, mouseSelectionButton: 2 no longer works. On 5.11.0, this option made the left button move/pan the map and the right button drag a selection box. On 5.11.2+ neither gesture does anything.

To Reproduce

1. Create a MindElixir instance with mouseSelectionButton: 2 and editable: true.
2. Left-drag on an empty area of the canvas; the map does not pan.
3. Right-drag on an empty area of the canvas; no selection box appears.

Expected behavior
Left button pans the map; right button draws a selection box (as in 5.11.0).

Additional context
mouseSelectionButton: 0 (the default) works as expected. The regression was likely introduced by the pointer state machine refactor in v5.11.2 (commit 04d8b9ee4f, "implement CanvasPointerDown state to coordinate canvas interaction and selection").

As a workaround we changed the canvas pointer-down branch in src/mouse.ts (https://github.com/SSShooter/mind-elixir-core/blob/master/src/mouse.ts#L220) to check e.mouseSelectionButton instead of a hardcoded left button, which restores the 5.11.0 behavior for us:

- if (mind.editable && target.className === 'map-container' && e.button === 0 && e.pointerType === 'mouse') {
+ if (mind.editable && target.className === 'map-container' && e.button === e.mouseSelectionButton && e.pointerType === 'mouse') {
   mind.ptState = State.BoxSelect
   return

There might be unknown side-effects as I'm not fully aware of the setup.

Start in src/mouse.ts around the canvas pointer-down branch at the linked location, then compare the pointer state machine refactor from commit 04d8b9ee4f with the reported workaround. Reproduce with mouseSelectionButton: 2 and editable: true; done means left-drag pans the map and right-drag draws a selection box without breaking the default setting.