feat(home): Songs-like → dedicated row + wider spread (#1491)

Promote the best-performing surface ("Songs like {artist}", ~8% skip /
~86% completion) out of the shared Playlists carousel into its own Home
row on both Android and web, and widen the daily build from 3 to 6 mixes
so the dedicated row shows a wider spread.

Server (internal/playlists):
- PickSeedArtists candidate pool 5 → 12; pickSeedArtistsForDay now takes
  songsLikeSeedCount (6) instead of a hardcoded 3. Graceful degradation
  and daily rotation preserved.

Android (HomeScreen.kt):
- New songsLikeSection + buildSongsLikeRow; PlaylistsRow takes a title so
  it renders both the "Playlists" and "Songs like…" rows. buildOnlineRow
  / orderedRealPlaylists no longer reserve the 3 songs-like slots.
  Offline shows cached mixes (available-first), hides the row when none.

Web (+page.svelte):
- Dedicated "Songs like…" row from songsLikeRow; dropped the 3-slot cap
  and removed songs-like from the Playlists carousel.

Tests: seed_selection_test.go, BuildPlaylistsRowTest.kt, page.test.ts.

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

android/app/src/main/java/com/fabledsword/minstrel/home/ui/HomeScreen.kt

@@ -623,10 +623,12 @@ private fun HomeSuccessContent(
     ) {
         item {
             // Always rendered: real system/user playlists, with
-            // placeholder cards filling the For You / Discover /
-            // 3× Songs-like slots that haven't generated yet. When
-            // offline, the cache-backed pool cards lead the row.
+            // placeholder cards filling the For You / Discover slots
+            // that haven't generated yet. When offline, the cache-backed
+            // pool cards lead the row. Songs-like now lives in its own
+            // dedicated row below (#1491), no longer inside this carousel.
             PlaylistsRow(
+                title = "Playlists",
                 rowItems = buildPlaylistsRow(sections.playlists, systemStatus, offline),
                 offline = offline,
                 onPlaylistClick = onPlaylistClick,
@@ -634,6 +636,18 @@ private fun HomeSuccessContent(
                 onPlayPlaylist = onPlayPlaylist,
             )
         }
+        // Songs-like is the best-performing surface (#1491) — promoted out
+        // of the Playlists carousel into its own row so it shows a wider
+        // spread of "Songs like {artist}" mixes. Hidden when there's
+        // nothing to show (offline with none cached).
+        songsLikeSection(
+            playlists = sections.playlists,
+            status = systemStatus,
+            offline = offline,
+            onPlaylistClick = onPlaylistClick,
+            onPlayPool = onPlayPool,
+            onPlayPlaylist = onPlayPlaylist,
+        )
         youMightLikeSection(
             albums = sections.youMightLikeAlbums,
             artists = sections.youMightLikeArtists,
@@ -657,6 +671,35 @@ private fun HomeSuccessContent(
     }
 }
 
+/**
+ * The dedicated "Songs like…" row (#1491). Reuses [PlaylistsRow]'s card
+ * rendering with a distinct title; hidden entirely when there's nothing
+ * to show (offline with no cached mixes). Online with none generated yet
+ * still shows a few placeholders so the building / seed-needed state is
+ * visible, matching the pre-promotion carousel behavior.
+ */
+private fun LazyListScope.songsLikeSection(
+    playlists: List<PlaylistRef>,
+    status: SystemPlaylistsStatus,
+    offline: Boolean,
+    onPlaylistClick: (String) -> Unit,
+    onPlayPool: (OfflinePoolKind) -> Unit,
+    onPlayPlaylist: suspend (PlaylistRef) -> Unit,
+) {
+    val rowItems = buildSongsLikeRow(playlists, status, offline)
+    if (rowItems.isEmpty()) return
+    item {
+        PlaylistsRow(
+            title = "Songs like…",
+            rowItems = rowItems,
+            offline = offline,
+            onPlaylistClick = onPlaylistClick,
+            onPlayPool = onPlayPool,
+            onPlayPlaylist = onPlayPlaylist,
+        )
+    }
+}
+
 private fun LazyListScope.recentlyAddedSection(
     albums: List<HomeTile<AlbumRef>>,
     onAlbumClick: (String) -> Unit,
@@ -926,13 +969,14 @@ private fun AlbumsRow(
 
 @Composable
 private fun PlaylistsRow(
+    title: String,
     rowItems: List<PlaylistRowItem>,
     offline: Boolean,
     onPlaylistClick: (String) -> Unit,
     onPlayPool: (OfflinePoolKind) -> Unit,
     onPlayPlaylist: suspend (PlaylistRef) -> Unit,
 ) {
-    HorizontalScrollRow(title = "Playlists") {
+    HorizontalScrollRow(title = title) {
         itemsIndexed(items = rowItems) { _, item ->
             when (item) {
                 is PlaylistRowItem.OfflinePool -> OfflinePoolCard(
@@ -983,11 +1027,12 @@ enum class OfflinePoolKind(val label: String) {
 /**
  * Builds the Home Playlists row.
  *
- * Online: For You + Discover + 3× Songs-like fixed slots (real card when
- * generated, placeholder otherwise), then the secondary system kinds (deep cuts
- * / rediscover / new for you / on this day / first listens) when they exist —
+ * Online: For You + Discover fixed slots (real card when generated,
+ * placeholder otherwise), then the secondary system kinds (deep cuts /
+ * rediscover / new for you / on this day / first listens) when they exist —
  * no placeholders for these since they're conditional on library shape — then
- * user-owned playlists.
+ * user-owned playlists. Songs-like has its own dedicated row (#1491) via
+ * [buildSongsLikeRow] and no longer appears in this carousel.
  *
  * Offline: the two cache-backed pools (Recently played, Liked) lead, then the
  * same real playlists in curated order but stably partitioned fully-cached
@@ -1014,7 +1059,7 @@ internal fun buildPlaylistsRow(
     return out
 }
 
-/** The online layout: fixed system slots (with placeholders), secondary, user. */
+/** The online layout: For You + Discover slots (with placeholders), secondary, user. */
 private fun buildOnlineRow(
     owned: List<PlaylistRef>,
     status: SystemPlaylistsStatus,
@@ -1026,12 +1071,7 @@ private fun buildOnlineRow(
     out += owned.firstOrNull { it.systemVariant == "discover" }
         ?.let { PlaylistRowItem.Real(it) }
         ?: PlaylistRowItem.Placeholder("Discover", variantFor("discover", status))
-    val songsLike = owned.filter { it.systemVariant == "songs_like_artist" }.take(SONGS_LIKE_SLOTS)
-    for (i in 0 until SONGS_LIKE_SLOTS) {
-        out += songsLike.getOrNull(i)
-            ?.let { PlaylistRowItem.Real(it) }
-            ?: PlaylistRowItem.Placeholder("Songs like…", variantFor("songs-like", status))
-    }
+    // Songs-like is no longer here — it has its own dedicated row (#1491).
     for (variant in SECONDARY_SYSTEM_VARIANTS) {
         owned.firstOrNull { it.systemVariant == variant }?.let { out += PlaylistRowItem.Real(it) }
     }
@@ -1039,16 +1079,42 @@ private fun buildOnlineRow(
     return out
 }
 
+/**
+ * Builds the dedicated Songs-like row (#1491): all "Songs like {artist}"
+ * mixes the server generated, no longer capped to the 3 carousel slots.
+ *
+ * Online: every generated mix as a real card; when none exist yet, a few
+ * placeholders so the building / seed-needed state stays visible.
+ * Offline: the cached mixes only (fully-cached first, greyed after), and
+ * an empty list — hiding the whole section — when nothing is cached.
+ */
+internal fun buildSongsLikeRow(
+    owned: List<PlaylistRef>,
+    status: SystemPlaylistsStatus,
+    offline: Boolean,
+): List<PlaylistRowItem> {
+    val mixes = owned.filter { it.systemVariant == "songs_like_artist" }
+    if (offline) {
+        val (available, greyed) = mixes.partition { !it.unavailableOffline }
+        return (available + greyed).map { PlaylistRowItem.Real(it) }
+    }
+    if (mixes.isNotEmpty()) return mixes.map { PlaylistRowItem.Real(it) }
+    return List(SONGS_LIKE_PLACEHOLDER_SLOTS) {
+        PlaylistRowItem.Placeholder("Songs like…", variantFor("songs-like", status))
+    }
+}
+
 /**
  * Curated real-playlist order (system primaries, then secondary, then user).
  * Must mirror [buildOnlineRow]'s slot order — the offline row reuses this and
  * only differs by dropping placeholders + partitioning available-first.
+ * Songs-like is excluded here too — it renders in its own row via
+ * [buildSongsLikeRow] in both online and offline modes (#1491).
  */
 private fun orderedRealPlaylists(owned: List<PlaylistRef>): List<PlaylistRef> {
     val out = mutableListOf<PlaylistRef>()
     owned.firstOrNull { it.systemVariant == "for_you" }?.let { out += it }
     owned.firstOrNull { it.systemVariant == "discover" }?.let { out += it }
-    out += owned.filter { it.systemVariant == "songs_like_artist" }.take(SONGS_LIKE_SLOTS)
     for (variant in SECONDARY_SYSTEM_VARIANTS) {
         owned.firstOrNull { it.systemVariant == variant }?.let { out += it }
     }
@@ -1063,7 +1129,10 @@ private fun variantFor(slot: String, s: SystemPlaylistsStatus): String = when {
     else -> "pending"
 }
 
-private const val SONGS_LIKE_SLOTS = 3
+// How many "Songs like…" placeholder cards the dedicated row shows while
+// the mixes haven't generated yet (building / seed-needed). Real mixes,
+// once generated, are shown in full and no longer capped by this (#1491).
+private const val SONGS_LIKE_PLACEHOLDER_SLOTS = 3
 
 /**
  * The 5 system playlist kinds the server generates that aren't pinned

android/app/src/test/java/com/fabledsword/minstrel/home/ui/BuildPlaylistsRowTest.kt

@@ -56,4 +56,51 @@ class BuildPlaylistsRowTest {
         assertTrue(row.none { it is PlaylistRowItem.OfflinePool })
         assertTrue(row.any { it is PlaylistRowItem.Placeholder })
     }
+
+    private fun songsLike(id: String, cached: Boolean) = PlaylistRef(
+        id = id,
+        userId = "u",
+        name = "Songs like $id",
+        systemVariant = "songs_like_artist",
+        trackCount = 25,
+        fullyCached = cached,
+    )
+
+    @Test
+    fun `songs-like no longer appears in the main playlists carousel`() {
+        val owned = listOf(songsLike("a", cached = true), user("u1", cached = true))
+        val row = buildPlaylistsRow(owned, SystemPlaylistsStatus(), offline = false)
+        val reals = row.filterIsInstance<PlaylistRowItem.Real>().map { it.playlist.id }
+        assertTrue("a" !in reals) { "songs-like mix leaked into the Playlists row: $reals" }
+        assertTrue("u1" in reals)
+    }
+
+    @Test
+    fun `online songs-like row shows every generated mix uncapped`() {
+        // Six generated mixes — the old carousel capped at 3; the dedicated row shows all.
+        val owned = (1..6).map { songsLike("a$it", cached = true) }
+        val row = buildSongsLikeRow(owned, SystemPlaylistsStatus(), offline = false)
+        val reals = row.filterIsInstance<PlaylistRowItem.Real>().map { it.playlist.id }
+        assertEquals((1..6).map { "a$it" }, reals)
+    }
+
+    @Test
+    fun `online songs-like row shows placeholders when none generated`() {
+        val row = buildSongsLikeRow(emptyList(), SystemPlaylistsStatus(), offline = false)
+        assertTrue(row.isNotEmpty())
+        assertTrue(row.all { it is PlaylistRowItem.Placeholder })
+    }
+
+    @Test
+    fun `offline songs-like row shows cached mixes available-first, none hides it`() {
+        val owned = listOf(songsLike("partial", cached = false), songsLike("full", cached = true))
+        val row = buildSongsLikeRow(owned, SystemPlaylistsStatus(), offline = true)
+        val reals = row.filterIsInstance<PlaylistRowItem.Real>().map { it.playlist.id }
+        // Fully-cached songs-like mix (not refreshable) is available; the
+        // un-cached one greys and sorts after. No placeholders offline.
+        assertEquals(listOf("full", "partial"), reals)
+        assertTrue(row.none { it is PlaylistRowItem.Placeholder })
+
+        assertTrue(buildSongsLikeRow(emptyList(), SystemPlaylistsStatus(), offline = true).isEmpty())
+    }
 }