bug: clear stale eraser cursor after scrolling or engine reassignment

[markm39]

Problem

After scrolling or native engine reassignment, the eraser cursor can remain visible on a page/panel even when the eraser is no longer actively being used. Visually, this looks like a stale eraser circle stuck on another canvas panel.

Expected behavior

The eraser cursor should only be visible during active eraser interaction. Scrolling, engine reassignment, tool changes, and gesture end/cancel paths should clear any stale cursor state.

Notes

Likely areas to inspect:

• Eraser pointer end/cancel handling.
• Native engine pool page reassignment.
• Tool changes while the viewport is moving.
• Cursor visibility state when a pooled native view is reused for another page.

Acceptance criteria

• No eraser circle remains visible after scrolling stops.
• No eraser circle remains visible after switching tools.
• No eraser circle leaks across pooled engine page assignment.