fix(Board): always drop the selection on an in-widget control press - #1364

Fixes CUB-3827.

selectionCancel promises that pressing an interactive descendant of a widget drops the board selection, but the reset rode on the widget host's bubble-phase onPointerDown — so it only happened for presses that actually reached the host. A control that calls stopPropagation() first never got there, which is why a widget's gear/leaf button dropped the selection while the chart's mapping button and an inline delete button silently left it standing.

What changed

• WidgetHost — the reset moved out of handleSelectPointerDown into two capture-phase handlers that only read the target (nothing is stopped or prevented, so every control keeps its press, focus and default behaviour):
  • a React onPointerDownCapture on the host. Capture runs top-down, so it lands before any descendant can call stopPropagation(), and because React propagates along the React tree it also covers a control the widget rendered into a portal (a menu opened from a widget's toolbar).
  • a native capture listener on the host node. React's capture phase is dispatched from the React root, so a descendant that stops the native event — charting and mapping libraries attach their own listeners and do exactly that — keeps it from reaching React at all. A listener on the host node itself sits below any such descendant. The two overlap in the ordinary case at no cost: clearing an already-empty selection is a no-op in commit.
• Board — the marquee no longer starts from a press on content a widget portaled outside the board. React events arrive from portals along the React tree, so a press in a popover over the board used to be able to open a lasso behind it.
• Docs line for selectionCancel now states the guarantee holds however the control handles the press.

Tests

Three regression specs under selection > clearing > however the press reaches the widget, one per way a press can miss the host: React stopPropagation, native stopPropagation, portaled control. All three fail on main and pass here.

pnpm test (2110 passed) and pnpm test:browser (127 passed) are green.

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