fix: cancel only the tapped occurrence on single-instance delete (#47)

"Delete only this event" on a recurring series wrote a cancelled
exception carrying just ORIGINAL_INSTANCE_TIME + STATUS_CANCELED. Without
DTSTART + DURATION the provider clones the master *with its RRULE intact*
and cancels the whole clone, so every other occurrence vanished, the
target survived as a "cancelled" ghost, and re-deleting toggled the
series back — exactly the reported corruption.

Anchor the exception as a single instance (DTSTART + DURATION + zone +
all-day, read from the series row) so the provider clears the inherited
RRULE and cancels only that occurrence — the same discipline the edit
path already documents (Codeberg #16). Also filter STATUS_CANCELED out of
the instances grid query so the cancelled occurrence disappears instead
of lingering as a tappable ghost (NULL status is kept — a normal event).

Extracts the exception ContentValues into a pure buildOccurrenceCancelValues
helper with JVM tests.

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

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

@@ -220,6 +220,46 @@ class EventWriteMapperTest {
         assertThat(values).containsEntry(CalendarContract.Events.DESCRIPTION, null)
     }
 
+    // --- buildOccurrenceCancelValues ("delete only this event") ---
+
+    @Test
+    fun `occurrence cancel anchors a single instance and cancels only it`() {
+        val values = buildOccurrenceCancelValues(
+            originalInstanceMillis = 1_700_000_000_000L,
+            dtStartMillis = 1_700_000_000_000L,
+            duration = "P3600S",
+            timezone = "Europe/Berlin",
+            allDay = 0,
+        )
+        assertThat(values[CalendarContract.Events.ORIGINAL_INSTANCE_TIME])
+            .isEqualTo(1_700_000_000_000L)
+        // DTSTART + DURATION make the provider derive a single instance and drop
+        // the inherited RRULE, so only this occurrence is cancelled — not the
+        // whole series (#47). DTEND is never sent (the provider rejects it).
+        assertThat(values[CalendarContract.Events.DTSTART]).isEqualTo(1_700_000_000_000L)
+        assertThat(values[CalendarContract.Events.DURATION]).isEqualTo("P3600S")
+        assertThat(values[CalendarContract.Events.EVENT_TIMEZONE]).isEqualTo("Europe/Berlin")
+        assertThat(values[CalendarContract.Events.STATUS])
+            .isEqualTo(CalendarContract.Events.STATUS_CANCELED)
+        assertThat(values).doesNotContainKey(CalendarContract.Events.DTEND)
+        assertThat(values).doesNotContainKey(CalendarContract.Events.RRULE)
+    }
+
+    @Test
+    fun `all-day occurrence cancel keeps the all-day flag and utc zone`() {
+        val values = buildOccurrenceCancelValues(
+            originalInstanceMillis = 1_700_000_000_000L,
+            dtStartMillis = 1_700_000_000_000L,
+            duration = "P1D",
+            timezone = "UTC",
+            allDay = 1,
+        )
+        assertThat(values[CalendarContract.Events.ALL_DAY]).isEqualTo(1)
+        assertThat(values[CalendarContract.Events.EVENT_TIMEZONE]).isEqualTo("UTC")
+        assertThat(values[CalendarContract.Events.STATUS])
+            .isEqualTo(CalendarContract.Events.STATUS_CANCELED)
+    }
+
     // --- per-event colour ---
 
     @Test