Scope render demand to each scene attachment

Give each scene its own render governor, owner holds and recent idle requests.
Make the Cesium render backend own its request mode directly, so the governor
drives engine.render on both engines without a dependency back from Cesium.
Route layer requests through their attached engine and capture the attachment's
governor for camera motions, orbit, annotations and shell controls. Release
retained-layer holds before clearing the outgoing engine, and destroy the
governor after scene consumers have detached.

Preserve Cesium's owner-keyed holds, pre-install buffering, last-release settling
frame and one-shot requests during continuous rendering. Navara keeps the same
render policy. There is no intended visible change on either engine. Simultaneous
scenes no longer share holds, and an old captured service cannot release a
replacement attachment's hold. Cover both rules on fake and headless Cesium
engines, including an old orbit ending after reattachment.

Viewer-reference lines fall from 910 to 891, Navara-kind checks from 2 to 1,
and engine-name lines from 33 to 32. All other coupling counts stay unchanged.

Checked in the browser, keyless: qa-perf passes on both engines with the
same idle, active and release counts as before (24 checks on Cesium, 22 on
Navara).

docs/GLOBE-PRIMITIVES.md

@@ -547,6 +547,24 @@ the same services on first use. The application destroys them on detach and
 creates fresh services when reattaching a parked engine. Identical layer IDs in
 simultaneous scenes cannot replace or unregister one another's picks.
 
+## Render demand
+
+`scene.services.render` owns continuous-render holds and the last 16 idle
+request reasons for one attachment. `holdContinuousRender(id)` and
+`releaseContinuousRender(id)` are idempotent per owner; the last release
+requests one settling frame. `governorRequestRender(reason)` forwards a
+one-shot request even while another owner holds continuous rendering.
+The tools call `install()` after scene consumers have registered their holds.
+
+The governor drives `engine.render` on every engine. Cesium implements
+`setContinuous` with its scene's `requestRenderMode` and disables automatic
+simulation-time renders with `maximumRenderTimeChange = Infinity`. Engine
+internals request their own scene directly and never import the governor.
+Layer service helpers take their attached engine as the final argument;
+attachment-bound callbacks can capture `renderGovernorFor(engine)` directly.
+Destroying the scene clears its holds and makes the captured service inert.
+A parked engine gets a fresh service when reattached.
+
 ## Render effects
 
 The optional `visualStyles`, `bloom` and `contactSonar` capabilities are