Test layers on one engine and share selection and shader helpers

Layer display tests no longer re-check Cesium primitives in every layer:
they run on the fake engine, and one Cesium translation check per ground
kind lives in the Cesium engine's ground tests. Their harnesses are shared,
and the fake-engine, Cesium-engine and flights tests that repeated the
contract suite are gone, 340 tests in all.

The press-travel measurement is shared by the selection dispatcher and
CCTV, which loses its ScreenSpaceEventHandler shim and world-click module.
ALPR, installations and bikeshare share one selection keeper and selected
marker write. Layers with their own click rules keep their handlers.

Navara's pulse is a class like the ground pass, the ground and imagery
passes share the surface uniforms, and the colour, log-depth and occlusion
GLSL lives in one chunk module, each caller keeping its depth bias. The
ground suite runs inside the contract suite (--only=ground:), and fixture
and script names say what they test.

Behaviour is unchanged on both engines.

scripts/qa-radio-engine-switch.mjs

@@ -1,5 +1,5 @@
 import assert from 'node:assert/strict';
-import { stations, installRadioFixture } from './fixtures/w2/radio.mjs';
+import { stations, installRadioFixture } from './fixtures/layers/radio.mjs';
 import { createStrict, localPreview, openPage, waitForEngine } from './qa-strict-harness.mjs';
 const url = localPreview(process.argv[2]);
 // The long soak and switch timings live in qa-strict-soak.mjs.