fix(edit): write occurrence exceptions with DURATION, not DTEND

Saving "only this event" on a recurring event inserts a modified-occurrence
exception at Events.CONTENT_EXCEPTION_URI. buildOccurrenceExceptionValues set
the occurrence's end as DTEND, but the provider rejects that outright —
CalendarProvider2.checkAllowedInException throws

    IllegalArgumentException: Exceptions can't overwrite dtend

so the insert failed, the save surfaced as SaveUiState.Failed, and the edit
screen reappeared with no change applied. "This and following" and "all events"
never go through the exception insert, which is why only "only this event" broke
(Codeberg #16, verified on a Pixel 10 / Android 16).

An exception is a single instance whose end the provider derives from
DTSTART + DURATION (clearing the inherited RRULE itself), so carry the length as
DURATION and drop DTEND — the same shape AOSP Calendar/Etar use. Verified
on-device: editing one occurrence now applies to just that occurrence and leaves
the rest of the series untouched.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/test/java/de/jeanlucmakiola/calendula/data/calendar/EventWriteMapperTest.kt

@@ -199,11 +199,12 @@ class EventWriteMapperTest {
         assertThat(values[CalendarContract.Events.TITLE]).isEqualTo("Moved")
         assertThat(values[CalendarContract.Events.EVENT_LOCATION]).isEqualTo("Berlin")
         assertThat(values[CalendarContract.Events.DTSTART]).isEqualTo(1_781_164_800_000L)
-        assertThat(values[CalendarContract.Events.DTEND])
-            .isEqualTo(1_781_164_800_000L + 5_400_000L)
-        // A single occurrence never carries its own rule.
+        // The occurrence's length travels as DURATION, never DTEND — the provider
+        // rejects DTEND on an exception ("Exceptions can't overwrite dtend") and
+        // derives the end from DTSTART + DURATION, clearing the inherited RRULE.
+        assertThat(values[CalendarContract.Events.DURATION]).isEqualTo("P5400S")
+        assertThat(values).doesNotContainKey(CalendarContract.Events.DTEND)
         assertThat(values).doesNotContainKey(CalendarContract.Events.RRULE)
-        assertThat(values).doesNotContainKey(CalendarContract.Events.DURATION)
     }
 
     @Test