M54: harden operations and demo resilience

frontend/e2e/greeting.spec.ts

@@ -1,5 +1,6 @@
 import { expect, test } from "@playwright/test";
 import { getBrusselsHour, getGreetingPeriod } from "../src/i18n/greeting";
+import { brusselsLocalToIso, toBrusselsDate, tryBrusselsLocalToIso } from "../src/i18n/brusselsDateTime";
 
 // Pure Node-context boundary tests for the central, clock-injectable greeting function
 // (section 9 of the Fleet Ops final localization brief). Every case below constructs an
@@ -66,3 +67,17 @@ test("default argument uses the real current time when no clock is injected", ()
   const period = getGreetingPeriod();
   expect(["morning", "afternoon", "evening", "night"]).toContain(period);
 });
+
+test("Brussels wall-clock conversion is DST-aware and rejects impossible local times", () => {
+  expect(brusselsLocalToIso("2026-01-15T12:00")).toBe("2026-01-15T11:00:00.000Z");
+  expect(brusselsLocalToIso("2026-07-15T12:00")).toBe("2026-07-15T10:00:00.000Z");
+  expect(tryBrusselsLocalToIso("2026-03-29T02:30")).toBeNull();
+  expect(tryBrusselsLocalToIso("2026-02-30T12:00")).toBeNull();
+  // The repeated autumn hour resolves deterministically to the post-transition CET instant.
+  expect(brusselsLocalToIso("2026-10-25T02:30")).toBe("2026-10-25T01:30:00.000Z");
+});
+
+test("Brussels calendar dates do not depend on the browser or runner timezone", () => {
+  expect(toBrusselsDate(new Date("2026-01-01T23:30:00.000Z"))).toBe("2026-01-02");
+  expect(toBrusselsDate(new Date("2026-07-01T22:30:00.000Z"))).toBe("2026-07-02");
+});