perf(video): warm up ExoPlayer pool, tune LoadControl, fix notification fall-through

Three small infrastructure fixes that support the eager-prepare model
(every visible feed video calls setMediaItem + prepare immediately so
it's ready when the user scrolls to it).

- ExoPlayerPool.create(): the warmup that builds poolStartingSize
  players up front was already written but never invoked. Wire it from
  PlaybackService.lazyPool() the first time a pool is requested. The
  builds now run on the pool's main-looper scope with a yield() between
  each so they're spread across frames instead of stalling the UI in
  one ~150–600 ms burst. Idempotent via an AtomicBoolean.

- ExoPlayerBuilder: install a feed-tuned DefaultLoadControl
  (10s/15s/750ms/2000ms) instead of the 50s/50s/2.5s/5s defaults. Every
  visible video preloads, so 5 simultaneous players were each trying to
  buffer 50s ahead — fighting for network and burning ~30 MB of buffer
  per HD player. Capping at 15s keeps the active video smooth, lets it
  start playing as soon as ~750 ms is buffered, and slashes peak memory
  on feeds with several preloads.

- PlaybackService.onUpdateNotification: the third forEachIndexed loop
  was missing its return, so on the muted-but-playing fallback path
  super.onUpdateNotification was called once per playing session
  instead of once total. With multiple feed videos preloading
  simultaneously this was hammering the notification system every time
  a player changed state. Match the first two loops by returning after
  the first match. Also drop the unused `idx` from forEachIndexed.

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/playerPool/ExoPlayerBuilder.kt

@@ -24,6 +24,7 @@ import android.content.Context
 import androidx.annotation.OptIn
 import androidx.media3.common.util.UnstableApi
 import androidx.media3.datasource.DataSource
+import androidx.media3.exoplayer.DefaultLoadControl
 import androidx.media3.exoplayer.ExoPlayer
 import com.vitorpamplona.amethyst.model.MediaAspectRatioCache
 import com.vitorpamplona.amethyst.service.playback.diskCache.VideoCache
@@ -42,10 +43,35 @@ class ExoPlayerBuilder(
             .Builder(context)
             .apply {
                 setMediaSourceFactory(CustomMediaSourceFactory(videoCache, dataSourceFactory))
+                setLoadControl(feedTunedLoadControl())
             }.build()
             .apply {
                 addListener(AspectRatioCacher(MediaAspectRatioCache))
                 addListener(KeepVideosPlaying(this))
                 addListener(CurrentPlayPositionCacher(this, VideoViewedPositionCache))
             }
+
+    companion object {
+        // Default DefaultLoadControl buffers 50s ahead before slowing down. Every visible video
+        // in the feed is prepared eagerly so it's ready when the user scrolls to it; with the
+        // default settings 5 simultaneous preloads would fight for ~250s of buffer between them
+        // and chew through ~30+ MB per HD player. Feed playback is optimized for "the active
+        // video plays smoothly while a few neighbours stay warm," so we cap the buffer at ~15s
+        // and let playback kick in as soon as ~750 ms is buffered. Fullscreen still gets a
+        // healthy buffer because seeks-within-15s are virtually instant from disk cache.
+        private fun feedTunedLoadControl() =
+            DefaultLoadControl
+                .Builder()
+                .setBufferDurationsMs(
+                    // minBufferMs =
+                    10_000,
+                    // maxBufferMs =
+                    15_000,
+                    // bufferForPlaybackMs =
+                    750,
+                    // bufferForPlaybackAfterRebufferMs =
+                    2_000,
+                ).setPrioritizeTimeOverSizeThresholds(true)
+                .build()
+    }
 }

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/playerPool/ExoPlayerPool.kt

@@ -34,7 +34,9 @@ import kotlinx.coroutines.cancel
 import kotlinx.coroutines.launch
 import kotlinx.coroutines.sync.Mutex
 import kotlinx.coroutines.sync.withLock
+import kotlinx.coroutines.yield
 import java.util.concurrent.ConcurrentLinkedQueue
+import java.util.concurrent.atomic.AtomicBoolean
 
 @OptIn(UnstableApi::class)
 class ExoPlayerPool(
@@ -54,9 +56,26 @@ class ExoPlayerPool(
 
     private val mutex = Mutex()
 
+    // Guards against firing the warmup more than once if create() is called repeatedly
+    // (e.g. on reconfiguration or when both pools share a startup hook).
+    private val warmupStarted = AtomicBoolean(false)
+
+    /**
+     * Pre-warms the pool with [poolStartingSize] ExoPlayer instances on the main looper, yielding
+     * between each build so the warmup is spread across frames instead of stalling the UI in one
+     * burst. ExoPlayer must be constructed on the same thread that will operate it (the main
+     * thread for this pool), so we cannot fan out across IO threads here. Idempotent — additional
+     * calls are no-ops.
+     */
     fun create(context: Context) {
-        while (playerPool.size < poolStartingSize) {
-            playerPool.offer(builder.build(context))
+        if (!warmupStarted.compareAndSet(false, true)) return
+        scope.launch {
+            while (playerPool.size < poolStartingSize) {
+                playerPool.offer(builder.build(context))
+                // Hand the frame back so an in-flight onGetSession / acquirePlayer / layout
+                // pass isn't blocked behind the next build.
+                yield()
+            }
         }
     }
 

amethyst/src/main/java/com/vitorpamplona/amethyst/service/playback/service/PlaybackService.kt

@@ -105,7 +105,15 @@ class PlaybackService : MediaSessionService() {
             val blossomServerResolver = Amethyst.instance.blossomResolver
 
             // creates new
-            return newPool(videoCache, okHttpClient, blossomServerResolver).also { poolNoProxy = it }
+            return newPool(videoCache, okHttpClient, blossomServerResolver)
+                .also {
+                    poolNoProxy = it
+                    // Kick off the player pool warmup as soon as we know this pool is being used.
+                    // It runs async on the main looper, yielding between builds, so the very first
+                    // session still acquires synchronously while subsequent ones can grab a warm
+                    // ExoPlayer instead of paying the build cost on the main thread.
+                    it.exoPlayerPool.create(applicationContext)
+                }
         } else {
             poolWithProxy?.let { return it }
 
@@ -116,7 +124,11 @@ class PlaybackService : MediaSessionService() {
             val videoCache = Amethyst.instance.videoCache
             val blossomServerResolver = Amethyst.instance.blossomResolver
 
-            return newPool(videoCache, okHttpClient, blossomServerResolver).also { poolWithProxy = it }
+            return newPool(videoCache, okHttpClient, blossomServerResolver)
+                .also {
+                    poolWithProxy = it
+                    it.exoPlayerPool.create(applicationContext)
+                }
         }
     }
 
@@ -161,23 +173,27 @@ class PlaybackService : MediaSessionService() {
             return
         }
 
-        playing.forEachIndexed { idx, it ->
+        playing.forEach {
             if (it.session.player.isPlaying && it.session.player.volume > 0 && it.session.id == BackgroundMedia.bgInstance?.id) {
                 super.onUpdateNotification(it.session, startInForegroundRequired)
                 return
             }
         }
 
-        playing.forEachIndexed { idx, it ->
+        playing.forEach {
             if (it.session.player.isPlaying && it.session.player.volume > 0) {
                 super.onUpdateNotification(it.session, startInForegroundRequired)
                 return
             }
         }
 
-        playing.forEachIndexed { idx, it ->
+        // Falls through to the first muted-but-playing session. Earlier this loop missed
+        // its return and called super.onUpdateNotification once per playing session,
+        // hammering the notification system whenever multiple feed videos were preloading.
+        playing.forEach {
             if (it.session.player.isPlaying) {
                 super.onUpdateNotification(it.session, startInForegroundRequired)
+                return
             }
         }
     }