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/data/dataCredits.test.mjs

@@ -244,3 +244,12 @@ test('Warsaw and Poznań credits carry the data date their licences ask for', ()
   assert.equal(mbta.key, 'transit-mbta');
   assert.doesNotMatch(mbta.html, /data of/);
 });
+
+test('the always-on rail credit names live feeds only, never timetables', () => {
+  // Every train shown is where its operator reported it; none is placed
+  // from a timetable.
+  const rail = DATA_CREDITS.find((entry) => entry.key === 'rail-gtfs-rt');
+  assert.ok(rail);
+  assert.match(rail.html, /realtime feeds/);
+  assert.doesNotMatch(rail.html, /timetable|placed from/i);
+});