Settle imagery, world data and sky capabilities in the renderer contract

- setFrames takes a timed source (URL frames, or video tiles in
  segments) on the scene clock by default or its own; frameReady fires
  once each time the shown frame is drawn.
- drapeOn is 'surface' | 'objects' | 'all' plus lit; imagery.host() and
  onHostChange go, and the drape rule Core derives is written out.
- ImagerySpec.fetchTile answers bytes, missing or defer on every
  renderer; tileTransform, admit and canvas imagery need imageHooks, and
  add() throws for them without it.
- Generic names for the default renderer's world data: worldDataToken,
  the photoreal fallback route, and world terrain and imagery.
- clouds and water capabilities, with globe.setClouds and setWater.

docs/RENDERER-CONTRACT.md

@@ -166,4 +166,6 @@ easing (`courseBlendMs`, `courseGlide`), and a hidden row keeping its eased
 course. The stub keeps no drawn course. These wait for scenes that capture
 an icon mid-turn. Height queries that count a set
 (`includeInHeightQueries`) are not checked either: the stub's heights are
-scripted, so it does not model them.
+scripted, so it does not model them. Nor are video frames (a check needs a
+video), `fetchTile` (the stub fetches nothing) or what clouds and water
+draw.