fix(store): three defects the instrumented suite found on first run

- forking an occurrence copied the master's alarm row id and hit the
  primary key; replaceForTask now clears it
- a sub-second DTSTART made lib-recur emit the anchor and its truncated
  self, doubling a series' first occurrence; floor to the second, which
  is all RFC 5545 DATE-TIME carries
- room-testing needs kotlinx-serialization 1.8+, but consistent
  resolution pinned androidTest to the app's 1.7.3

Tests: 52 pass on device.

app/src/main/java/de/jeanlucmakiola/agendula/data/tasks/room/TaskAlarmDao.kt

@@ -22,10 +22,14 @@ interface TaskAlarmDao {
     @Query("DELETE FROM task_alarms WHERE task_id = :taskId")
     fun deleteForTask(taskId: Long): Int
 
-    /** Set the task's only reminder, or clear it with `null`. */
+    /**
+     * Set the task's only reminder, or clear it with `null`. The row that lands is
+     * always a new one — the id is cleared so an alarm lifted off another task
+     * (forking an occurrence copies the master's) inserts instead of colliding.
+     */
     @Transaction
     fun replaceForTask(taskId: Long, alarm: TaskAlarmEntity?) {
         deleteForTask(taskId)
-        alarm?.let { insert(it.copy(taskId = taskId)) }
+        alarm?.let { insert(it.copy(id = 0, taskId = taskId)) }
     }
 }

app/src/main/java/de/jeanlucmakiola/agendula/domain/recurrence/RecurrenceExpander.kt

@@ -9,6 +9,7 @@ import java.time.ZoneId
 import java.util.TimeZone
 import kotlin.time.Instant
 
+private const val MILLIS_PER_SECOND = 1000L
 private const val MILLIS_PER_DAY = 24L * 60 * 60 * 1000
 
 /** The rule set of one task series, as stored. All strings are raw iCalendar values. */
@@ -117,10 +118,17 @@ object RecurrenceExpander {
         return TimeZone.getTimeZone(stored ?: floatingZone)
     }
 
-    /** All-day series are date-anchored: pin the anchor to UTC midnight, as it is stored. */
+    /**
+     * All-day series are date-anchored: pin the anchor to UTC midnight, as it is
+     * stored. A timed one is floored to the second, because RFC 5545 DATE-TIME
+     * has no sub-second field — carrying millis in makes lib-recur emit the raw
+     * anchor *and* its truncated self, doubling the first occurrence, and mints
+     * `RECURRENCE-ID`s no other client could address.
+     */
     private fun anchorMillis(spec: RecurrenceSpec): Long {
         val millis = spec.anchor.toEpochMilliseconds()
-        return if (!spec.isAllDay) millis else Math.floorDiv(millis, MILLIS_PER_DAY) * MILLIS_PER_DAY
+        val unit = if (spec.isAllDay) MILLIS_PER_DAY else MILLIS_PER_SECOND
+        return Math.floorDiv(millis, unit) * unit
     }
 
     private fun ruleOf(value: String, zone: TimeZone): RecurrenceRule? = runCatching {