fix(agenda): stop yesterday's all-day event surfacing under today (#65)

The agenda window starts at the anchor's local midnight, which east of UTC
is the previous day's 22:00 UTC. All-day events live at UTC midnights with an
exclusive next-midnight end, so yesterday's all-day event (a birthday, say)
still overlaps today's window start and is returned by the provider.
groupAgendaDays then clamped its first day up to the anchor and, via a trailing
coerceAtLeast(firstDay) on the last day, pulled it onto the anchor's "today"
section — the multi-day fix only stopped the forward leak on interior days.

Drop instances whose true last day (resolved in UTC for all-day events) falls
before the anchor, or whose first day falls past the window end: they occupy no
visible day and must not be clamped onto an edge. Adds eastern-zone regression
tests for both edges.

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

app/src/main/java/de/jeanlucmakiola/calendula/ui/agenda/AgendaUiState.kt

@@ -94,10 +94,16 @@ fun groupAgendaDays(
     val byDay = sortedMapOf<LocalDate, MutableList<EventInstance>>()
     for (instance in instances) {
         val firstDay = instance.spanFirstDay(zone).coerceAtLeast(anchor)
-        // Never below firstDay: an instance the query returned always surfaces on
-        // at least its first visible day, even if its end resolves earlier (e.g. a
-        // zero-length or boundary instant) — otherwise the loop would drop it.
-        val lastDay = instance.spanLastDay(zone).coerceAtMost(windowEnd).coerceAtLeast(firstDay)
+        val lastDay = instance.spanLastDay(zone).coerceAtMost(windowEnd)
+        // Skip instances that don't actually occupy any day in [[anchor], [windowEnd]].
+        // The provider returns an event whenever its instant span overlaps the query
+        // window, but all-day events live at UTC midnights with an exclusive end: east
+        // of UTC that end dips just past local midnight, so *yesterday's* all-day event
+        // overlaps today's window start and comes back even though its true last day
+        // (resolved in UTC) is before the anchor. Clamping it up to the anchor would
+        // surface it under "today" (issue #65); drop it instead. The symmetric case —
+        // a next-day all-day event overlapping the window's last instant — drops too.
+        if (lastDay < firstDay) continue
         var day = firstDay
         while (day <= lastDay) {
             byDay.getOrPut(day) { mutableListOf() }.add(instance)