Add detectStationChange helper for multi-leg timeline

src/features/online-board/components/FullRouteTimeline/detectStationChange.test.ts

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+import { describe, it, expect } from "vitest";
+import { detectStationChange } from "./detectStationChange.js";
+import type { IFlightLegStation } from "../../types.js";
+
+function makeStation(overrides: {
+  cityCode?: string;
+  airportCode?: string;
+  terminal?: string;
+}): IFlightLegStation {
+  return {
+    scheduled: {
+      airport: "",
+      airportCode: overrides.airportCode ?? "SVO",
+      city: "",
+      cityCode: overrides.cityCode ?? "MOW",
+      countryCode: "RU",
+    },
+    latest: {
+      airport: "",
+      airportCode: overrides.airportCode ?? "SVO",
+      city: "",
+      cityCode: overrides.cityCode ?? "MOW",
+      countryCode: "RU",
+    },
+    dispatch: "",
+    gate: "",
+    terminal: overrides.terminal ?? "",
+  } as IFlightLegStation;
+}
+
+describe("detectStationChange", () => {
+  it("returns 'noChange' when city/airport/terminal all match", () => {
+    const a = makeStation({ cityCode: "MOW", airportCode: "SVO", terminal: "D" });
+    const b = makeStation({ cityCode: "MOW", airportCode: "SVO", terminal: "D" });
+    expect(detectStationChange(a, b)).toBe("noChange");
+  });
+
+  it("returns 'city' when cityCode differs", () => {
+    const a = makeStation({ cityCode: "MOW", airportCode: "SVO" });
+    const b = makeStation({ cityCode: "LED", airportCode: "LED" });
+    expect(detectStationChange(a, b)).toBe("city");
+  });
+
+  it("returns 'airport' when cities match but airport differs", () => {
+    const a = makeStation({ cityCode: "MOW", airportCode: "SVO" });
+    const b = makeStation({ cityCode: "MOW", airportCode: "DME" });
+    expect(detectStationChange(a, b)).toBe("airport");
+  });
+
+  it("returns 'terminal' when airport matches but terminal differs", () => {
+    const a = makeStation({ cityCode: "MOW", airportCode: "SVO", terminal: "D" });
+    const b = makeStation({ cityCode: "MOW", airportCode: "SVO", terminal: "F" });
+    expect(detectStationChange(a, b)).toBe("terminal");
+  });
+
+  it("returns 'noChange' when terminals are both empty strings", () => {
+    const a = makeStation({ cityCode: "MOW", airportCode: "SVO", terminal: "" });
+    const b = makeStation({ cityCode: "MOW", airportCode: "SVO", terminal: "" });
+    expect(detectStationChange(a, b)).toBe("noChange");
+  });
+});

src/features/online-board/components/FullRouteTimeline/detectStationChange.ts

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+import type { IFlightLegStation } from "../../types.js";
+
+export type StationChange = "city" | "airport" | "terminal" | "noChange";
+
+/**
+ * Detect the kind of transition between two stations by priority:
+ * city change > airport change > terminal change > no change.
+ *
+ * Uses the `scheduled` snapshot for comparisons (stable across updates).
+ */
+export function detectStationChange(
+  from: IFlightLegStation,
+  to: IFlightLegStation,
+): StationChange {
+  if (from.scheduled.cityCode !== to.scheduled.cityCode) return "city";
+  if (from.scheduled.airportCode !== to.scheduled.airportCode) return "airport";
+  if (from.terminal && to.terminal && from.terminal !== to.terminal) return "terminal";
+  return "noChange";
+}