test(06-01): add failing tests for end-tracking duration math

- computeNewTimedEnd: 1h delta, 26h multi-day delta, floor-to-1h rule
- computeNewAllDayEnd: 0-day span, 3-day span, floor-to-same-day rule
- RED: all 6 new cases fail with missing-export TypeError

apps/pwa/src/lib/eventDateTime.test.ts

@@ -13,7 +13,7 @@
  */
 
 import { describe, it, expect } from 'vitest'
-import { serializeEventDateTime, localWallClockToUtcIso } from './eventDateTime.js'
+import { serializeEventDateTime, localWallClockToUtcIso, computeNewTimedEnd, computeNewAllDayEnd } from './eventDateTime.js'
 
 describe('serializeEventDateTime (BUG A — write-path TZ)', () => {
   it('serializes a timed start to a UTC instant (ends in Z)', () => {
@@ -51,3 +51,64 @@ describe('serializeEventDateTime (BUG A — write-path TZ)', () => {
     expect(new Date(iso).getHours()).toBe(9)
   })
 })
+
+describe('computeNewTimedEnd (D-04 — end-tracking)', () => {
+  it('preserves a 1-hour timed delta when start moves forward', () => {
+    // Old: 2026-06-11 09:00 → 10:00 (1h duration)
+    // New start: 2026-06-11 11:00 → new end should be 12:00
+    const result = computeNewTimedEnd(
+      '2026-06-11', '11:00',
+      '2026-06-11', '09:00',
+      '2026-06-11', '10:00',
+    )
+    expect(result.endDate).toBe('2026-06-11')
+    expect(result.endTime).toBe('12:00')
+  })
+
+  it('preserves a multi-day timed delta (26h) when start moves', () => {
+    // Old: 2026-06-11 08:00 → 2026-06-12 10:00 (26h duration)
+    // New start: 2026-06-13 08:00 → new end should be 2026-06-14 10:00
+    const result = computeNewTimedEnd(
+      '2026-06-13', '08:00',
+      '2026-06-11', '08:00',
+      '2026-06-12', '10:00',
+    )
+    expect(result.endDate).toBe('2026-06-14')
+    expect(result.endTime).toBe('10:00')
+  })
+
+  it('floors to 1h when old end was already behind old start', () => {
+    // Old end (10:00) <= old start (11:00) — stale/invalid state
+    // New start: 2026-06-11 14:00 → new end should snap to +1h = 15:00
+    const result = computeNewTimedEnd(
+      '2026-06-11', '14:00',
+      '2026-06-11', '11:00',
+      '2026-06-11', '10:00',
+    )
+    expect(result.endDate).toBe('2026-06-11')
+    expect(result.endTime).toBe('15:00')
+  })
+})
+
+describe('computeNewAllDayEnd (D-04 — all-day end-tracking)', () => {
+  it('preserves a 0-day span (single-day all-day event)', () => {
+    // Old: 2026-06-11 → 2026-06-11 (0-day span, single day)
+    // New start: 2026-06-15 → new end should be 2026-06-15
+    const result = computeNewAllDayEnd('2026-06-15', '2026-06-11', '2026-06-11')
+    expect(result).toBe('2026-06-15')
+  })
+
+  it('preserves a 3-day span when start moves', () => {
+    // Old: 2026-06-11 → 2026-06-14 (3-day span)
+    // New start: 2026-06-20 → new end should be 2026-06-23
+    const result = computeNewAllDayEnd('2026-06-20', '2026-06-11', '2026-06-14')
+    expect(result).toBe('2026-06-23')
+  })
+
+  it('floors to same day when old end was already behind old start', () => {
+    // Old end (2026-06-10) < old start (2026-06-11) — stale/invalid state
+    // New start: 2026-06-15 → new end should snap to same day 2026-06-15
+    const result = computeNewAllDayEnd('2026-06-15', '2026-06-11', '2026-06-10')
+    expect(result).toBe('2026-06-15')
+  })
+})