Plugins only received PointerMoved events while the pointer was inside
their rect, so a hover state that ran to the edge of the plugin stayed
active after the pointer moved out. Add InputEvent::PointerLeft, send it
from the host when the pointer moves out, leaves the window, enters a
hole, or a drag that left the viewport is released, and map it to
PointerGone in the beui and egui guest paths.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012U1gB8zGp5FFAcTqyrZdD4