Reuse shipped Engine code

Before writing behaviour, check what the installed playcanvas package already provides.

Scripts live under scripts/esm/**; import only from there. List them:

rg 'static scriptName =' node_modules/playcanvas/scripts/esm \
  | sed "s|.*/scripts/esm/||; s|:.*static scriptName = ['\"]|  ->  |; s|['\"].*||" \
  | sort

Common ones are CameraControls, FirstPersonController, ThirdPersonController, ShadowCatcher, ProceduralSky, Water, and Grid, plus XR and splat scripts. Modules under scripts/esm/parsers are resource-handler classes, not scripts.

Core exports that are not scripts include CameraFrame for post-processing, Picker (getSelectionAsync), TransformGizmo, OutlineRenderer, MiniStats, GltfExporter, and UsdzExporter.