fix(alarms): let the backstop always close the ring it was armed for

Fourth round of review fixes for the silence-after setting.

onAutoSilence no longer re-arms a ring it measures as inside the window.
After a clock moved back, that check could push the backstop out again
and stretch the ring well past its window. Every writer of the window and
every clock change already re-arms the backstop under the engine lock, so
a backstop that fires has had its window, and it closes the ring as in v1.

A one-shot alarm the resolver switches off as missed also clears a
pending skip flag, as the resolver's other one-shot disable does.
SNOOZE_GRACE is its own ten-minute constant again, not tied to the
setting's default.

Refs #17

app/src/main/java/de/jeanlucmakiola/clockula/alarm/AlarmEngine.kt

@@ -31,7 +31,6 @@ import kotlinx.coroutines.flow.map
 import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
 import kotlinx.coroutines.withContext
-import kotlin.time.Duration
 import kotlin.time.Duration.Companion.minutes
 import kotlin.time.Instant
 import javax.inject.Inject
@@ -83,15 +82,12 @@ class AlarmEngine @Inject constructor(
 
     suspend fun onAutoSilence(alarmId: Long): Unit = lock.withLock {
         val state = states.state(alarmId)
-        val since = state.ringingSince ?: return@withLock
+        if (state.ringingSince == null) return@withLock
 
-        // A backstop armed for a shorter window than the one now chosen ("Silence
-        // after" was raised mid-ring): the ring still has time, so the backstop
-        // moves rather than ending it early. A negative age is a clock moved back
-        // behind the ring's start; the backstop was then armed a window from that
-        // moved clock, so the ring has had its window and closes.
-        val age = wallClock.now() - since
-        if (age >= Duration.ZERO && rearmIfInsideWindowLocked(alarmId, since)) return@withLock
+        // The backstop is always armed for the window in force: every writer of
+        // "Silence after" and every clock change re-arms it under this lock. So
+        // when it fires, the ring has had its window, whatever the wall clock
+        // now says about its age.
 
         // Missed, not snoozed: the ring stops, the cycle closes, the next
         // occurrence is scheduled. There is no missed-alarm notification in v1.
@@ -377,7 +373,7 @@ class AlarmEngine @Inject constructor(
      * run out ⇒ false, and the caller closes or drops it. Measured from when the
      * ring started, so neither a reboot nor a setting change buys it a fresh
      * window — or from now, if the clock has since moved back behind that
-     * start, so a clock change cannot stretch a ring past one window either.
+     * start, so a clock moved back adds at most the time already rung.
      */
     private suspend fun rearmIfInsideWindowLocked(alarmId: Long, since: Instant): Boolean {
         val now = wallClock.now()

app/src/main/java/de/jeanlucmakiola/clockula/domain/alarm/AlarmResolver.kt

@@ -107,6 +107,7 @@ object AlarmResolver {
                 nextFire = null,
                 source = FireSource.NONE,
                 state = working.copy(skippedOccurrence = null),
+                clearSkipFlag = alarm.skipNextOccurrence,
                 disableAlarm = true,
             )
         }

app/src/main/java/de/jeanlucmakiola/clockula/domain/alarm/AlarmRing.kt

@@ -1,6 +1,5 @@
 package de.jeanlucmakiola.clockula.domain.alarm
 
-import de.jeanlucmakiola.clockula.domain.ring.SilenceAfter
 import kotlin.time.Duration
 import kotlin.time.Duration.Companion.minutes
 
@@ -24,7 +23,7 @@ object AlarmRing {
      * window or this, whichever is longer, so a short window never makes a
      * promised snooze easier to lose.
      */
-    val SNOOZE_GRACE: Duration = SilenceAfter.DEFAULT_MINUTES.minutes
+    val SNOOZE_GRACE: Duration = 10.minutes
 
     /** How late a snooze may come due and still ring, given the ring window. */
     fun snoozeGrace(silenceAfter: Duration): Duration = maxOf(silenceAfter, SNOOZE_GRACE)

app/src/test/java/de/jeanlucmakiola/clockula/alarm/AlarmEngineTest.kt

@@ -912,23 +912,6 @@ class AlarmEngineTest {
         assertThat(entry.source).isEqualTo(FireSource.RESUMED_RING)
     }
 
-    @Test
-    fun `a backstop armed for a shorter window moves rather than firing early`(@TempDir dir: Path) = runTest {
-        givenAlarm(repeatDays = RepeatDays.EVERY_DAY)
-        wallClock.instant = sevenToday
-        val settings = settings(dir)
-        val engine = engineOver(settings)
-        engine.onFire(alarmId = 1L, scheduledFor = sevenToday)
-        settings.setAlarmSilenceAfterMinutes(25)
-        wallClock.instant = sevenToday + 10.minutes
-
-        engine.onAutoSilence(1L)
-
-        assertThat(ring.ringing.value).isEqualTo(1L)
-        assertThat(states.state(1L).ringingSince).isEqualTo(sevenToday)
-        assertThat(scheduler.autoSilence).isEqualTo(Registration(sevenToday + 25.minutes, 1L))
-    }
-
     @Test
     fun `raising silence-after mid-ring moves the backstop at once`(@TempDir dir: Path) = runTest {
         givenAlarm(repeatDays = RepeatDays.EVERY_DAY)