Give each engine its own extensions in place of the navaraViews map

What only one engine has now travels with the engine instead of a
WeakMap in navaraMode.js: engine.extensions.navara holds Navara's view,
its SDK classes and the daylight reset, engine.extensions.cesium the
Cesium viewer. The contract documents the field as outside the contract,
and the coupling report counts its reads outside src/globe in place of
the navaraViews reads (27, unchanged).

scripts/engine-coupling-baseline.json

@@ -6,7 +6,7 @@
     "viewerLines": 2424,
     "navaraKindChecks": 17,
     "engineSniffCalls": 51,
-    "navaraViewsReads": 27,
+    "engineExtensionReads": 27,
     "groundEffectPasses": 9,
     "cesiumSdkLoads": 6,
     "screenSpaceHandlerFiles": 20,