## Summary
Adds `offsetX`, `offsetY`, `movementX`, `movementY` to
`SerializedEventData` and the host event serialiser so apps can reason
about element-relative pointer positions without trying to read the host
element's bounding rect (which is impossible from a remote-DOM worker).
## Motivation
I was building a front-component with click-to-drop-pin and trackpad
pan/zoom (custom OSM tile renderer). Two real bugs surfaced from the
current event-serialisation surface:
1. **Wheel pan/zoom was broken.** The host already forwards
`deltaX`/`deltaY`, but app authors naturally read them off the
React-style event handler argument as `e.deltaX`/`e.deltaY`. Because
remote-DOM bridges everything via `RemoteEvent extends
CustomEvent<Detail>`, the payload actually arrives at `e.detail.deltaX`.
Reading the wrong place gives `undefined`, and `undefined < 0 ===
false`, so every wheel notch zoomed in the same direction. App code now
uses `e.detail`, but this was a sharp papercut worth flagging in docs /
a helper (separate change).
2. **Element-local click coords are unobtainable from a worker.** With
only `clientX/Y`, an app needs the stage's bounding rect to translate
viewport coordinates to local — which can't be read across the worker
boundary. `offsetX`/`offsetY` close that gap with a one-read solution.
`movementX`/`movementY` round out the set for any future drag-style
interactions if `mousemove` later joins the allow-list.
