Gate out-of-window jump-to-unread on genuinely unread displayable content

The jump-to-unread FAB could surface an OutOfWindow target when the
fully-read marker event was either loaded-but-not-displayed (state/filtered
events) or when there was nothing displayable to jump to. Add two guards to
the recompute:

- Require roomInfo.numUnreadMessages > 0, so the FAB only appears when there
  is genuinely unread "interesting" content (never state/hidden events).
- Add Timeline.isEventLoaded to distinguish "in the loaded window but not
  rendered" from "genuinely outside the window", falling back to the SDK only
  after the cheap display-index check (isKnown) misses.

The RustTimeline implementation reads the replay cache first and releases the
EventTimelineItem native handle via use {} when probing the SDK. Adds presenter
tests for the loaded-but-not-displayed and no-unread-messages branches.

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

libraries/matrix/test/src/main/kotlin/io/element/android/libraries/matrix/test/timeline/FakeTimeline.kt

@@ -436,6 +436,9 @@ class FakeTimeline(
 
     override suspend fun loadReplyDetails(eventId: EventId) = loadReplyDetailsLambda(eventId)
 
+    var isEventLoadedLambda: (eventId: EventId) -> Boolean = { false }
+    override suspend fun isEventLoaded(eventId: EventId): Boolean = isEventLoadedLambda(eventId)
+
     var pinEventLambda: (eventId: EventId) -> Result<Boolean> = { lambdaError() }
     override suspend fun pinEvent(eventId: EventId): Result<Boolean> {
         return pinEventLambda(eventId)