feat(rail): add a Rail layer with keyless live trains

Live trains from 30 keyless operator feeds: GPS positions from 29 and
Irish Rail's station reports, shown as the operators publish them. 28
work out of the box; Renfe long and medium distance and KTMB appear once
their pack is built, because their vehicles name no route.

Packs are opt-in (scripts/build-rail-paths.mjs): the operator's own
route lines, stop names, next stops and trip-update delays for live
trains. Nothing downloads when the app starts, no train is placed from a
timetable, and no request carries a deployer's key.

src/app/layers/rail.js

@@ -0,0 +1,50 @@
+import { createRailLayer } from '../../layers/rail/index.js';
+import * as render from '../../renderGovernor.js';
+import * as sprites from '../../data/spriteOrder.js';
+import * as picking from '../../data/pickRegistry.js';
+import * as overlays from '../../overlays/worldOverlay.js';
+import { applyTrackedCameraFrame } from '../../data/trackedCamera.js';
+import { WORLD_FOCUS_FRAMING, requestWorldFocus } from '../../worldFocus.js';
+import {
+  railFeedCredit,
+  registerDynamicCredit,
+} from '../../data/dataCredits.js';
+
+/**
+ * Construct the Rail layer on the application's scene owners and its shared
+ * ground floor, the same floor Transit and the aircraft stand on.
+ * @param {{surface: object, source?: object}} options
+ * @returns {object}
+ */
+export function createApplicationRail({ surface, source }) {
+  if (!surface?.groundFloor)
+    throw new TypeError('Rail needs the application surface services');
+  const { groundFloor } = surface;
+  return createRailLayer({
+    source,
+    services: {
+      render,
+      sprites,
+      picking,
+      overlays,
+      credits: { registerDynamicCredit, railFeedCredit },
+      ground: {
+        GROUND_FLOOR_LIFT_M: groundFloor.GROUND_FLOOR_LIFT_M,
+        cachedGroundFloor: groundFloor.cachedGroundFloor,
+        warmGroundFloor: groundFloor.warmGroundFloor,
+        neighborFloorM: groundFloor.neighborFloorM,
+        coarseFloorCoord: groundFloor.coarseFloorCoord,
+      },
+      // A selected train is followed as a tracked aircraft is, or flown to
+      // once as a vessel is (src/layers/rail/follow.js).
+      camera: {
+        framing: WORLD_FOCUS_FRAMING.train,
+        requestWorldFocus,
+        applyTrackedCameraFrame,
+        isCockpitActive: () =>
+          globalThis.document?.body?.classList?.contains('cockpit-mode') ===
+          true,
+      },
+    },
+  });
+}