refactor(overlays): layer labels through the engine

The world overlay host already projects through engine.projector and tests
the horizon with projector.horizonVisible; the last Cesium value on its
path, the snapshot of an entry's cullPosition, is now a geo Cartesian3.

Navara's horizonVisible is Cesium's EllipsoidalOccluder.isPointVisible:
tested against Cesium's own on 8 000 eye/point pairs (600 m to 20 000 km,
points under, on and over the ellipsoid; no disagreement) and at the limb
(a label 40 m under the ellipsoid just past the horizon is hidden, with its
cullPosition lifted 100 m it is shown). It ran per label per frame with
three new vectors each call; it now uses scratch vectors. Micro-measure
(500 labels per frame, no scavenge during the run): 7 941 B per frame
before, 51 after.

The W1 sources that projected with Cesium's SceneTransforms themselves -
the icon orientation's exact projection and the launch replay overlay - go
through the engine's projector when their scene or viewer has a registered
engine. Satellites, vessel labels and aircraft recession project nothing
themselves (the host does).

scripts/qa-overlay-engines.mjs publishes the same labels on both engines at
a city, an orbital and a limb pose and compares the painted rectangles:
worst 0 / 2 / 0 px; on both, the far-side label is not painted, the lifted
limb label is and the buried one is not.

scripts/format-scope.json

@@ -1095,5 +1095,6 @@
   "scripts/qa-surface-heights.mjs",
   "src/globe/navara/followGestures.js",
   "src/globe/navara/followGestures.test.mjs",
-  "src/globe/cpuPicks.test.mjs"
+  "src/globe/cpuPicks.test.mjs",
+  "scripts/qa-overlay-engines.mjs"
 ]