feat(globe): engine members and the viewer bridge for the shell

Contract: camera.position(result?) and camera.height() for per-frame
readers, render.request(reason?), maps.releasePhotorealTiles, sources
with a caller-owned ready provider, isGlobeEngine. engine.maps now hides
every tileset it has shown, lets the last imagery/photoreal call win, and
setPhotorealTiles(null) restores the last base imagery and its credit
instead of leaving the photoreal credit on a bare globe.

Cesium engine: reads the Viewer camera/canvas aliases, detaches bridged
events through the remover Cesium returns, raises frame timestamps
without allocating, re-aims the horizon occluder only when the camera
moved, and renders the scene directly when the governor governs another
viewer (or none yet). engineFromViewer returns the registered scene
engine of a viewer, else builds and caches one. The render governor stays
the Cesium render backend under engine.render and gains
isRenderGovernorInstalled.

scripts/package-boundaries.json

@@ -1751,6 +1751,7 @@
       "src/globe/cesium/projector.js",
       "src/globe/cesium/render.js",
       "src/globe/cesium/surface.js",
+      "src/globe/cesium/engineFromViewer.js",
       "src/sources/sourceSlot.js",
       "src/app/sources.js",
       "src/app/startupChrome.js",