feat(android,player): the read-ahead on the seek bar

pstr_mpv observes demuxer-cache-time and hands it over with the rest of
the state, and the seek bar shades from the start to where the demuxer
has read, lighter than the played part. A jump that lands in the shaded
run costs no fetch, as on the desktop.

android/app/src/main/java/io/narl/protonstream/playback/NativeMpvHost.kt

@@ -75,6 +75,8 @@ data class MpvPlaybackState(
     val cachePercent: Double = 0.0,
     /** A seek has been issued but playback has not resumed at the new position. */
     val seeking: Boolean = false,
+    /** How far the demuxer has read ahead, in seconds into the file. */
+    val cachedUntil: Double = 0.0,
 ) {
     /** Whether the picture is stopped for a reason the viewer did not choose. */
     val stalled: Boolean get() = (buffering || seeking) && !paused && !ended
@@ -313,6 +315,7 @@ class NativeMpvHost private constructor(private val nativeHandle: Long) : Libmpv
                         buffering = values[BUFFERING] != 0.0,
                         cachePercent = values[CACHE_PERCENT],
                         seeking = values[SEEKING] != 0.0,
+                        cachedUntil = values[CACHED_UNTIL],
                     )
                     mutableState.value = next
                     // The listeners drive the media session and the player UI,
@@ -431,7 +434,7 @@ class NativeMpvHost private constructor(private val nativeHandle: Long) : Libmpv
 
     companion object {
         /** Fields `nativeState` fills, in the order `pstr_mpv.cpp` writes them. */
-        private const val FIELDS = 13
+        private const val FIELDS = 14
         private const val GENERATION = 6
         private const val WIDTH = 7
         private const val HEIGHT = 8
@@ -439,6 +442,7 @@ class NativeMpvHost private constructor(private val nativeHandle: Long) : Libmpv
         private const val BUFFERING = 10
         private const val CACHE_PERCENT = 11
         private const val SEEKING = 12
+        private const val CACHED_UNTIL = 13
 
         init {
             System.loadLibrary("pstr_android")

android/app/src/main/java/io/narl/protonstream/playback/PlayerScreen.kt

@@ -640,7 +640,12 @@ private fun PlayerControls(
                         scrubbing = null
                     },
                     valueRange = 0f..duration,
-                    track = { AccentTrack(it) },
+                    track = {
+                        AccentTrack(
+                            it,
+                            buffered = if (state.duration > 0) (state.cachedUntil / state.duration).toFloat() else 0f,
+                        )
+                    },
                 )
                 ChapterMarks(chapters, state.duration, Modifier.matchParentSize())
             }