address the review: keep the stored seed on a failed decode, one cache, static bar colour

- resolve() no longer discards a stored seed when the bitmap cannot be decoded (it used to
  return null, dropping the theme for pre-v12 rows); it falls back to the stored pair
- fold the busy flag into the seed cache value so eviction is atomic (a busy entry could be
  evicted from the separate flag LRU while its seed stayed, silently disabling the scrim)
- the artist top bar samples heroBarColor at scroll 0 to match the static HeroGradient, and
  drops the wrong scroll estimate and per-pixel recomposition
- rememberOverlaidNameBusy keys its state so the cached flag is read on key change, not only
  the first composition; bottomBandBusy bails out on an empty bitmap

app/src/main/kotlin/com/subtracks/ui/library/ArtistDetailScreen.kt

@@ -151,14 +151,7 @@ fun ArtistDetailScreen(
             BoxWithConstraints(modifier.fillMaxSize().background(Color.Black)) {
                 val screenHeightPx = with(density) { maxHeight.toPx() }
                 val barHeightPx = with(density) { barHeight.toPx() }
-                val scrollPx by remember {
-                    derivedStateOf {
-                        val index = listState.firstVisibleItemIndex
-                        val before = if (index <= 0) 0f else with(density) { ART_HEIGHT.toPx() } * index
-                        before + listState.firstVisibleItemScrollOffset
-                    }
-                }
-                val barColor = heroBarColor(artwork, scrollPx, barHeightPx, screenHeightPx)
+                val barColor = heroBarColor(artwork, 0f, barHeightPx, screenHeightPx)
                 HeroGradient(
                     colors = artwork,
                     scrollPx = { 0f },