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/railScheduleIndex.js

@@ -0,0 +1,124 @@
+/**
+ * @module railScheduleIndex
+ * @description Server-side lookup over a schedule pack (the
+ * `<feedId>.schedule.json` scripts/build-rail-paths.mjs writes) for a
+ * realtime source that names a train by its number (Irish Rail's train
+ * codes): `createRunIndex`, the dated runs of a pack's trips, so a reported
+ * train takes the trip it is on (its line and stops). The train's position
+ * is never taken from the timetable.
+ *
+ * Pure: no I/O. Times are epoch milliseconds; a run starts at the service
+ * day's GTFS origin (noon minus 12 h in the pack's time zone) plus the
+ * trip's start, and lasts its profile's last offset, as
+ * src/data/railSchedule.js reads them.
+ */
+
+import { serviceDayOriginMs } from './railTimeZones.js';
+
+function packTrips(raw) {
+  return raw?.trips && typeof raw.trips === 'object' ? raw.trips : {};
+}
+
+function packServices(raw) {
+  return raw?.services && typeof raw.services === 'object' ? raw.services : {};
+}
+
+function serviceDates(services, service) {
+  const dates = services[service];
+  return Array.isArray(dates) ? dates : [];
+}
+
+/**
+ * The dated runs of a pack's trips. Built lazily per trip, so a source that
+ * looks up a few hundred train numbers never expands the whole pack. It
+ * keeps the parts of the pack it reads (trips, profiles, services), not the
+ * pack itself.
+ * @param {object} raw Decoded schedule pack.
+ * @returns {{runsOf: (tripId: string) => Array<{tripId: string, date: string,
+ *     startMs: number, endMs: number}>,
+ *   tripsNamed: (name: string) => string[],
+ *   activeTrip: (name: string, nowMs: number,
+ *     options?: {slackMs?: number, delayMs?: number}) =>
+ *     ({tripId: string, date: string, startMs: number, endMs: number}|null)}}
+ */
+export function createRunIndex(raw) {
+  const trips = packTrips(raw);
+  const services = packServices(raw);
+  const zone = typeof raw?.timezone === 'string' ? raw.timezone : 'UTC';
+  const profiles = Array.isArray(raw?.profiles) ? raw.profiles : [];
+  const origins = new Map();
+  const originOf = (date) => {
+    let origin = origins.get(date);
+    if (origin === undefined) {
+      origin = serviceDayOriginMs(date, zone);
+      origins.set(date, origin);
+    }
+    return origin;
+  };
+  let names = null;
+  const runs = new Map();
+  function runsOf(tripId) {
+    let list = runs.get(tripId);
+    if (list) return list;
+    list = [];
+    const trip = Object.hasOwn(trips, tripId) ? trips[tripId] : null;
+    const profile = trip ? profiles[trip.profile] : null;
+    if (trip && Array.isArray(profile) && Number.isFinite(trip.startSec)) {
+      const durationMs = (Number(profile[profile.length - 1]) || 0) * 1000;
+      for (const date of serviceDates(services, trip.service)) {
+        const origin = originOf(date);
+        if (!Number.isFinite(origin)) continue;
+        const startMs = origin + trip.startSec * 1000;
+        list.push({ tripId, date, startMs, endMs: startMs + durationMs });
+      }
+    }
+    runs.set(tripId, list);
+    return list;
+  }
+  return {
+    runsOf,
+    tripsNamed(name) {
+      if (!names) {
+        names = new Map();
+        for (const [tripId, trip] of Object.entries(trips)) {
+          if (typeof trip?.name !== 'string' || !trip.name) continue;
+          const list = names.get(trip.name);
+          if (list) list.push(tripId);
+          else names.set(trip.name, [tripId]);
+        }
+      }
+      return names.get(String(name ?? '')) ?? [];
+    },
+    /**
+     * The run of a train number that is under way at `nowMs` (or within
+     * `slackMs` of its start or end); the one whose span is nearest when
+     * several qualify, null when none or when two tie. A train running
+     * `delayMs` late (negative: early) is where its timetable was that long
+     * ago, so its runs are compared at `nowMs - delayMs`.
+     */
+    activeTrip(name, nowMs, { slackMs = 10 * 60_000, delayMs = 0 } = {}) {
+      const at = nowMs - (Number.isFinite(delayMs) ? delayMs : 0);
+      let best = null;
+      let bestScore = Infinity;
+      let tie = false;
+      for (const tripId of this.tripsNamed(name)) {
+        for (const run of runsOf(tripId)) {
+          if (at < run.startMs - slackMs || at > run.endMs + slackMs) continue;
+          const score =
+            at < run.startMs
+              ? run.startMs - at
+              : at > run.endMs
+                ? at - run.endMs
+                : 0;
+          if (score < bestScore) {
+            best = run;
+            bestScore = score;
+            tie = false;
+          } else if (score === bestScore && best && run.tripId !== best.tripId)
+            tie = true;
+        }
+      }
+      return tie ? null : best;
+    },
+  };
+}