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/fixtures/layers/combined.mjs

@@ -0,0 +1,93 @@
+const ORIGIN = { lat: 42.3554, lon: -71.0605 };
+const grid = (i, cols, spacing) => ({
+  lat: ORIGIN.lat + (Math.floor(i / cols) - cols / 2) * spacing,
+  lon: ORIGIN.lon + ((i % cols) - cols / 2) * spacing,
+});
+const bikes = Array.from({ length: 200 }, (_, i) => ({
+  station_id: `fixture-bike-${i}`,
+  name: `Fixture station ${i}`,
+  capacity: 20,
+  ...grid(i, 15, 0.0015),
+}));
+export const combinedPayloads = { bikes };
+
+/** Bikeshare and startup feed fixtures shared by both engines. */
+export async function installCombinedFixture(page) {
+  const requests = [];
+  const session = await page.createCDPSession();
+  await session.send('Fetch.enable', {
+    patterns: ['127.0.0.1', 'localhost'].map((host) => ({
+      urlPattern: `http://${host}:*/api/*`,
+    })),
+  });
+  let disposed = false;
+  session.on('Fetch.requestPaused', async (event) => {
+    try {
+      const u = new URL(event.request.url);
+      let body;
+      const p = u.pathname;
+      if (p.startsWith('/api/gbfs/'))
+        body = {
+          version: '3.0',
+          data: {
+            stations: decodeURIComponent(u.href).includes('station_information')
+              ? bikes
+              : bikes.map(({ station_id }) => ({
+                  station_id,
+                  num_bikes_available: 7,
+                  num_docks_available: 13,
+                  is_renting: true,
+                  is_returning: true,
+                })),
+          },
+        };
+      else if (p === '/api/flights') body = { states: [] };
+      else if (p === '/api/military') body = { ac: [] };
+      else if (p === '/api/vessels') body = { connected: false, vessels: [] };
+      else if (p === '/api/google/nearby-places') body = { places: [] };
+      else if (p === '/api/setup/status') body = { ready: true };
+      else if (p === '/api/terrain/heights')
+        body = {
+          results: (u.searchParams.get('points') || '')
+            .split(';')
+            .filter(Boolean)
+            .map((point) => {
+              const [lon, lat] = point.split(',').map(Number);
+              return { lon, lat, elevation: 20, geoid: 0, ellipsoid: 20 };
+            }),
+        };
+      if (body === undefined) {
+        await session.send('Fetch.continueRequest', {
+          requestId: event.requestId,
+        });
+        return;
+      }
+      requests.push(p);
+      await session.send('Fetch.fulfillRequest', {
+        requestId: event.requestId,
+        responseCode: 200,
+        responseHeaders: [
+          { name: 'Content-Type', value: 'application/json' },
+          { name: 'Access-Control-Allow-Origin', value: '*' },
+          { name: 'Access-Control-Expose-Headers', value: 'Data-Present' },
+          { name: 'Data-Present', value: 'true' },
+        ],
+        body: Buffer.from(JSON.stringify(body)).toString('base64'),
+      });
+    } catch (error) {
+      if (
+        !disposed &&
+        !page.isClosed() &&
+        !String(error.message).includes('Invalid InterceptionId')
+      )
+        page.emit('pageerror', error);
+    }
+  });
+  return {
+    requests,
+    async dispose() {
+      disposed = true;
+      await session.detach();
+    },
+  };
+}