Function initPicking

(Re-)build the GPU picking pipeline for the current scene.

1. Tear down any previous picking session (event listeners, system, loaders) so a dataset switch never leaks state.
2. Walk the freshly-loaded LuxarScene root looking for text/image channels and interaction templates. Bail out early when nothing can consume a pick.
3. Stand up label/key LabelLoader instances and an ImageLabelLoader backed by the scene loader's zarr store.
4. Construct PickingSystem with the pick-result handler that forwards to OverlayManager.updateHoverContent.
5. Wire DOM + Three.js EventDispatcher listeners (mousemove, mouseleave, sceneManager change, controls start/end, window resize, sceneManager 'camera-changed') through the supplied EventGroup so a future dispose() removes them in one call.

Returns the new system + loaders; the orchestrator stores all four on its own fields. The handler closures capture the locally-created pickingSystem so they always see the current instance (not a stale reference from a previous session).