Threaded mode: an event for the start and end of a live resize #63

Part of #49; follows #53. Found while building the renderer's side (sidorares/react-x11#542).

What

window-resize and resize-handshake both carry live, but nothing says when a live resize has ended. AppKit's tracking loop calls windowDidResize: once per pointer move, and calls nothing when the pointer stops or lifts.

The renderer needs the end. During a drag it lays out against the content floors it already has. Once the drag is over, it measures once and lays out again (react-x11 src/nodes/window/size.js, _deferContentFloors).

• In pump mode it read the end as "the pump ran at all": a pump tick means the tracking loop has let go of the thread.
• On a worker there is no such signal. react-x11's threaded mode now treats a 100 ms pause in the ticks as the end. That costs two ways:
  • a drag that pauses mid-way ends early, which costs one extra measured layout;
  • the catch-up after the release lands up to 100 ms late.

Proposed (mechanism only)

Emit windowWillStartLiveResize: and windowDidEndLiveResize: as window events, in both modes. For example window-live-resize { windowNumber, handle, phase: 'begin' | 'end' }; the bridge chooses the names.

Optionally, add liveResize to the published window state as well.

With that event, react-x11 replaces its timer with it.