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.

changelog.d/0-rail-layer.md

@@ -0,0 +1,24 @@
+- Add Rail in the Movement group, off by default: live trains from 30 keyless
+  operator feeds in the United States, Canada, Australia, Ireland, Finland,
+  Spain, Norway, Malaysia, Japan and France, from commuter and long-distance
+  rail to the streetcars and trams of Seattle, Cincinnati, Toyama, Bordeaux
+  and Montpellier. A new `/api/rail` proxy fetches registered feeds only,
+  needs no key and drops buses from mixed feeds. Every train shown is live:
+  29 feeds publish GPS positions and Irish Rail its station reports, and 28
+  of them show with no setup. No train is placed or moved from a timetable;
+  a feed whose live positions fail shows as unavailable. Packs are an opt-in
+  extra: `node scripts/build-rail-paths.mjs --feed <id>` (or `--all`) builds
+  a feed's path and schedule packs into `.cache/rail-paths/`, which add the
+  operator's own route lines, stop names, the next stop and trip-update
+  delays to its live trains; Renfe long and medium distance and KTMB, whose
+  vehicles name no route, appear once their pack is built. Nothing is
+  downloaded when the app starts, and lines are only ever the operator's own
+  shapes. Trains are coloured by category (urban, local, regional, national,
+  international), each with a chip and legend entry shared via token `02`,
+  glide between reports with Transit's delayed playback, are drawn once when
+  several feeds carry them, and open a card on click. A selected train that
+  is moving is followed by the camera, as an aircraft is, and any other is
+  flown to once; the selected train and its line stay on the map from orbit.
+  No train is placed in Ukraine. Feeds that publish no licence are used on
+  the basis each one's entry records, and their credit never presents that
+  as a licence (Denis Groshev, @ZloyS, #0).