perf(video): tighten remember keys and stabilize controller-overlay tree

Round-4 audit cleanups. Each item is small but each runs on the hot path
that recomposes during every active video, so they add up while scrolling.

P1 — DimensionTag identity invalidating remember:
- DimensionTag (in quartz) is a regular class with no equals override, so
  reference equality means a freshly parsed tag for the same event is !=
  to the previous one. The remember(videoUri, dimensions) blocks added in
  the earlier perf commits were re-running their lambda on every recompose.
  Switch to primitive (width, height) keys in VideoView and GifVideoView so
  the cache lookups + MediaAspectRatioCache writes only fire when the
  dimensions actually change.

P1 — Static gradient brushes:
- TopGradientOverlay / BottomGradientOverlay were calling
  Brush.verticalGradient(colors = colors) inside the modifier chain, which
  allocated a fresh Brush on every recomposition while the controllers
  were visible (i.e. on every active video most of the time). Pre-build
  both brushes as file-level vals so they're allocated exactly once per
  process.

P2 — ImmutableList for action collections:
- RenderTopButtons / AnimatedOverflowMenuButton / OverflowMenuButton were
  passing List<VideoPlayerAction> across composable boundaries. Plain List
  is unstable in Compose, forcing the overflow tree to recompose any time
  an unrelated parent state (volume, tracks, controllerVisible) ticked.
  Use ImmutableList end-to-end via toImmutableList() at the producer side.

P2 — videoPlayerButtonItemsFlow remember:
- accountViewModel.videoPlayerButtonItemsFlow() was being called fresh
  every recomposition, with the result handed straight to
  collectAsStateWithLifecycle. Hoist the call into remember(accountViewModel)
  so the flow reference is stable.

P2 — MuteButton dispatcher cleanup:
- The 2-second hold timer was using LaunchedEffect { launch(Dispatchers.IO)
  { delay(2000); holdOn.value = false } }. The wrapped launch was just
  redundant dispatcher hopping — delay() doesn't hold a thread and the
  Compose write is fine on Main. Inline it.

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/composable/VideoView.kt

@@ -115,13 +115,16 @@ fun VideoView(
 
     // Resolve the aspect ratio once per composition. Prime the URL-keyed cache from the imeta
     // dim tag so the next time this video appears (PiP, dialog, list re-enter) the cache hits
-    // without waiting for ExoPlayer's onVideoSizeChanged.
+    // without waiting for ExoPlayer's onVideoSizeChanged. Keys are primitive width/height so
+    // a freshly parsed DimensionTag instance for the same event doesn't re-run this lambda —
+    // DimensionTag uses reference equality, not structural.
+    val dimW = dimensions?.width
+    val dimH = dimensions?.height
     val ratio =
-        remember(videoUri, dimensions) {
-            val fromDim = dimensions?.takeIf { it.hasSize() }
-            if (fromDim != null) {
-                MediaAspectRatioCache.add(videoUri, fromDim.width, fromDim.height)
-                fromDim.aspectRatio()
+        remember(videoUri, dimW, dimH) {
+            if (dimW != null && dimH != null && dimW > 0 && dimH > 0) {
+                MediaAspectRatioCache.add(videoUri, dimW, dimH)
+                dimW.toFloat() / dimH.toFloat()
             } else {
                 MediaAspectRatioCache.get(videoUri)
             }

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/composable/controls/GradientOverlay.kt

@@ -38,24 +38,33 @@ import androidx.compose.ui.unit.dp
 private val FadeIn = fadeIn()
 private val FadeOut = fadeOut()
 
-private val TopGradientColors =
-    listOf(
-        Color.Black.copy(alpha = 0.6f),
-        Color.Black.copy(alpha = 0.3f),
-        Color.Transparent,
+// Both gradient brushes are static; pre-build them once at class init so we don't allocate a
+// new Brush on every recomposition while the controllers are visible (which is most of the
+// time during playback / interaction).
+private val TopGradientBrush =
+    Brush.verticalGradient(
+        colors =
+            listOf(
+                Color.Black.copy(alpha = 0.6f),
+                Color.Black.copy(alpha = 0.3f),
+                Color.Transparent,
+            ),
     )
 
-private val BottomGradientColors =
-    listOf(
-        Color.Transparent,
-        Color.Black.copy(alpha = 0.4f),
-        Color.Black.copy(alpha = 0.7f),
+private val BottomGradientBrush =
+    Brush.verticalGradient(
+        colors =
+            listOf(
+                Color.Transparent,
+                Color.Black.copy(alpha = 0.4f),
+                Color.Black.copy(alpha = 0.7f),
+            ),
     )
 
 @Composable
 private fun GradientOverlay(
     controllerVisible: State<Boolean>,
-    colors: List<Color>,
+    brush: Brush,
     height: Dp,
     modifier: Modifier = Modifier,
 ) {
@@ -70,7 +79,7 @@ private fun GradientOverlay(
                 Modifier
                     .fillMaxWidth()
                     .height(height)
-                    .background(brush = Brush.verticalGradient(colors = colors)),
+                    .background(brush = brush),
         )
     }
 }
@@ -80,11 +89,11 @@ fun TopGradientOverlay(
     controllerVisible: State<Boolean>,
     modifier: Modifier = Modifier,
     height: Dp = 80.dp,
-) = GradientOverlay(controllerVisible, TopGradientColors, height, modifier)
+) = GradientOverlay(controllerVisible, TopGradientBrush, height, modifier)
 
 @Composable
 fun BottomGradientOverlay(
     controllerVisible: State<Boolean>,
     modifier: Modifier = Modifier,
     height: Dp = 120.dp,
-) = GradientOverlay(controllerVisible, BottomGradientColors, height, modifier)
+) = GradientOverlay(controllerVisible, BottomGradientBrush, height, modifier)

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/composable/controls/MuteButton.kt

@@ -47,9 +47,7 @@ import com.vitorpamplona.amethyst.ui.theme.Size30Modifier
 import com.vitorpamplona.amethyst.ui.theme.Size50Modifier
 import com.vitorpamplona.amethyst.ui.theme.ThemeComparisonColumn
 import com.vitorpamplona.amethyst.ui.theme.VolumeBottomIconSize
-import kotlinx.coroutines.Dispatchers
 import kotlinx.coroutines.delay
-import kotlinx.coroutines.launch
 
 @Preview
 @Composable
@@ -79,11 +77,12 @@ fun MuteButton(
             )
         }
 
+    // LaunchedEffect already runs on Main, and delay() suspends without holding a thread, so
+    // the previous launch(Dispatchers.IO) was just unnecessary dispatcher hopping for a state
+    // mutation that's also fine on Main.
     LaunchedEffect(key1 = controllerVisible) {
-        launch(Dispatchers.IO) {
-            delay(2000)
-            holdOn.value = false
-        }
+        delay(2000)
+        holdOn.value = false
     }
 
     val mutedInstance = remember(startingMuteState) { mutableStateOf(startingMuteState) }

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/composable/controls/OverflowMenu.kt

@@ -50,6 +50,8 @@ import com.vitorpamplona.amethyst.ui.theme.Size20Modifier
 import com.vitorpamplona.amethyst.ui.theme.Size50Modifier
 import com.vitorpamplona.amethyst.ui.theme.ThemeComparisonColumn
 import com.vitorpamplona.amethyst.ui.theme.VolumeBottomIconSize
+import kotlinx.collections.immutable.ImmutableList
+import kotlinx.collections.immutable.persistentListOf
 
 private val FadeIn = fadeIn()
 private val FadeOut = fadeOut()
@@ -60,7 +62,7 @@ fun OverflowMenuButtonPreview() {
     ThemeComparisonColumn {
         Box(Modifier.background(BitcoinOrange)) {
             OverflowMenuButton(
-                actions = listOf(VideoPlayerAction.Share, VideoPlayerAction.Download, VideoPlayerAction.PictureInPicture),
+                actions = persistentListOf(VideoPlayerAction.Share, VideoPlayerAction.Download, VideoPlayerAction.PictureInPicture),
                 startingMuteState = false,
                 onFullscreenClick = {},
                 onMuteClick = {},
@@ -76,7 +78,7 @@ fun OverflowMenuButtonPreview() {
 @Composable
 fun AnimatedOverflowMenuButton(
     controllerVisible: State<Boolean>,
-    actions: List<VideoPlayerAction>,
+    actions: ImmutableList<VideoPlayerAction>,
     startingMuteState: Boolean,
     onFullscreenClick: (() -> Unit)?,
     onMuteClick: () -> Unit,
@@ -107,7 +109,7 @@ fun AnimatedOverflowMenuButton(
 
 @Composable
 fun OverflowMenuButton(
-    actions: List<VideoPlayerAction>,
+    actions: ImmutableList<VideoPlayerAction>,
     startingMuteState: Boolean,
     onFullscreenClick: (() -> Unit)?,
     onMuteClick: () -> Unit,

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/composable/controls/RenderTopButtons.kt

@@ -70,6 +70,7 @@ import com.vitorpamplona.amethyst.ui.theme.PinBottomIconSize
 import com.vitorpamplona.amethyst.ui.theme.Size20Modifier
 import com.vitorpamplona.amethyst.ui.theme.Size50Modifier
 import com.vitorpamplona.amethyst.ui.theme.ThemeComparisonColumn
+import kotlinx.collections.immutable.toImmutableList
 
 @Preview
 @Composable
@@ -194,7 +195,10 @@ fun RenderTopButtons(
     modifier: Modifier,
     accountViewModel: AccountViewModel,
 ) {
-    val buttonItems by accountViewModel.videoPlayerButtonItemsFlow().collectAsStateWithLifecycle()
+    // Hold the StateFlow itself across recompositions so collectAsStateWithLifecycle isn't
+    // keyed on the result of a property getter call that happens every recompose.
+    val buttonItemsFlow = remember(accountViewModel) { accountViewModel.videoPlayerButtonItemsFlow() }
+    val buttonItems by buttonItemsFlow.collectAsStateWithLifecycle()
     val shareDialogVisible = remember { mutableStateOf(false) }
     val saveAction =
         rememberSaveMediaAction { context ->
@@ -212,17 +216,22 @@ fun RenderTopButtons(
         }
 
     val canFullscreen = onZoomClick != null
+    // ImmutableList so Compose can treat the action lists as stable parameters when they're
+    // passed through to AnimatedOverflowMenuButton — a plain List is unstable and forces the
+    // overflow tree to recompose whenever any unrelated parent state ticks.
     val topBarActions =
         remember(buttonItems, canFullscreen, hasMultipleQualities, isLive, pipSupported) {
             buttonItems
                 .filter { it.location == VideoButtonLocation.TopBar && isAvailable(it.action) }
                 .map { it.action }
+                .toImmutableList()
         }
     val overflowActions =
         remember(buttonItems, canFullscreen, hasMultipleQualities, isLive, pipSupported) {
             buttonItems
                 .filter { it.location == VideoButtonLocation.OverflowMenu && isAvailable(it.action) }
                 .map { it.action }
+                .toImmutableList()
         }
 
     Row(modifier) {

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/components/GifVideoView.kt

@@ -35,6 +35,7 @@ import androidx.compose.runtime.LaunchedEffect
 import androidx.compose.runtime.SideEffect
 import androidx.compose.runtime.collectAsState
 import androidx.compose.runtime.getValue
+import androidx.compose.runtime.remember
 import androidx.compose.ui.Alignment
 import androidx.compose.ui.Modifier
 import androidx.compose.ui.draw.clip
@@ -68,7 +69,18 @@ fun GifVideoView(
     accountViewModel: AccountViewModel,
     thumbhash: String? = null,
 ) {
-    val ratio = dimensions?.aspectRatio() ?: MediaAspectRatioCache.get(videoUri)
+    // Keys are primitive width/height so a freshly parsed DimensionTag instance for the same
+    // event doesn't re-run this lambda — DimensionTag uses reference equality, not structural.
+    val dimW = dimensions?.width
+    val dimH = dimensions?.height
+    val ratio =
+        remember(videoUri, dimW, dimH) {
+            if (dimW != null && dimH != null && dimH > 0) {
+                dimW.toFloat() / dimH.toFloat()
+            } else {
+                MediaAspectRatioCache.get(videoUri)
+            }
+        }
     val autoPlay = accountViewModel.settings.autoPlayVideos()
     val borderModifier = if (roundedCorner) MaterialTheme.colorScheme.imageModifier else Modifier
     val context = LocalContext.current