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.

package.json

@@ -285,6 +285,11 @@
     "./layers/transit/standalone": "./src/data/transit.js",
     "./layers/transit/source": "./src/layers/transit/source.js",
     "./sources/transit-service": "./src/sources/transitService.js",
+    "./server/providers/rail": {
+      "node": "./server/providers/rail.js"
+    },
+    "./sources/rail-service": "./src/sources/railService.js",
+    "./layers/rail/source": "./src/layers/rail/source.js",
     "./sources/map-features": "./src/sources/overpassFeatures.js",
     "./server/providers/mapillary": {
       "node": "./server/providers/mapillary.js"