library: rank artist cover fallback like the materialized list

Reuse the materialized ranking CTE chain for the per-artist fallback so
duplicate covers keep the same album, exercise the four-row cap, and
test every credit path plus source isolation. Resolve the art input as a
pure function and key the artist and covers off the same source.

app/src/test/kotlin/com/subtracks/data/db/LibraryDaoTest.kt

@@ -656,7 +656,7 @@ class LibraryDaoTest {
         }
 
     @Test
-    fun coverArtFallbackFindsTrackCreditsWithoutAnArtistRow() =
+    fun coverArtFallbackFindsEveryCreditPath() =
         runTest {
             val sourceId = source()
             val dao = db.libraryDao()
@@ -696,9 +696,44 @@ class LibraryDaoTest {
             assertNull(dao.artistOnce(sourceId, "ar-missing"))
             assertNull(dao.artistOnce(sourceId, "ar-featured"))
             assertNull(dao.artistOnce(sourceId, "ar-guest"))
-            assertEquals(listOf("track-art"), dao.artistCoverArts(sourceId, "ar-missing"))
-            assertEquals(listOf("featured-art"), dao.artistCoverArts(sourceId, "ar-featured"))
-            assertEquals(listOf("featured-art"), dao.artistCoverArts(sourceId, "ar-guest"))
+            assertEquals(listOf("track-art"), dao.artistCoverArts(sourceId, "ar-missing").first())
+            assertEquals(listOf("featured-art", "track-art"), dao.artistCoverArts(sourceId, "ar-other").first())
+            assertEquals(listOf("featured-art"), dao.artistCoverArts(sourceId, "ar-featured").first())
+            assertEquals(listOf("featured-art"), dao.artistCoverArts(sourceId, "ar-guest").first())
+        }
+
+    @Test
+    fun artistCoverArtsRanksDeduplicatesCapsAndStaysInSource() =
+        runTest {
+            val sourceId = source()
+            val otherSourceId = source()
+            val dao = db.libraryDao()
+            dao.upsertArtists(listOf(artist(sourceId, "ar-rank", "Rank", albumCount = 0)))
+            dao.upsertAlbums(
+                listOf(
+                    album(sourceId, "a-new", "New", year = 2020, starred = null)
+                        .copy(artistId = "ar-rank", coverArt = "shared"),
+                    album(sourceId, "a-mid", "Mid", year = 2015, starred = null)
+                        .copy(artistId = "ar-rank", coverArt = "other"),
+                    album(sourceId, "a-old", "Old", year = 2010, starred = null)
+                        .copy(artistId = "ar-rank", coverArt = "shared"),
+                    album(sourceId, "a-4", "Four", year = 2012, starred = null)
+                        .copy(artistId = "ar-rank", coverArt = "c4"),
+                    album(sourceId, "a-5", "Five", year = 2011, starred = null)
+                        .copy(artistId = "ar-rank", coverArt = "c5"),
+                    album(sourceId, "a-6", "Six", year = 2009, starred = null)
+                        .copy(artistId = "ar-rank", coverArt = "c6"),
+                    album(otherSourceId, "a-leak", "Leak", year = 2025, starred = null)
+                        .copy(artistId = "ar-rank", coverArt = "leak"),
+                ),
+            )
+
+            val expected = listOf("shared", "other", "c4", "c5")
+            assertEquals(expected, dao.artistCoverArts(sourceId, "ar-rank").first())
+
+            dao.recomputeArtistArt(sourceId)
+
+            assertEquals(expected, dao.artistOnce(sourceId, "ar-rank")!!.coverArts)
         }
 
     @Test

app/src/test/kotlin/com/subtracks/ui/library/ArtistDetailArtTest.kt

@@ -0,0 +1,105 @@
+package com.subtracks.ui.library
+
+import com.subtracks.data.model.Artist
+import org.junit.Assert.assertEquals
+import org.junit.Assert.assertNull
+import org.junit.Test
+
+class ArtistDetailArtTest {
+    @Test
+    fun aCreditedArtistWithoutARowUsesTheCreditCovers() {
+        val resolved =
+            resolveArtistArtInput(
+                ArtistArtInput(
+                    artist = null,
+                    artistId = "ar1",
+                    offline = false,
+                    sourceId = 7,
+                    covers = listOf("a", "b"),
+                ),
+            )
+
+        assertEquals(7L, resolved?.sourceId)
+        assertEquals("ar1", resolved?.id)
+        assertEquals(listOf("a", "b"), resolved?.coverArts)
+    }
+
+    @Test
+    fun anOfflineArtistKeepsItsDownloadedCovers() {
+        val resolved =
+            resolveArtistArtInput(
+                ArtistArtInput(
+                    artist = artist(coverArts = listOf("online"), downloadedCoverArts = listOf("downloaded")),
+                    artistId = "ar1",
+                    offline = true,
+                    sourceId = 7,
+                    covers = listOf("online", "other"),
+                ),
+            )
+
+        assertEquals(listOf("online", "other"), resolved?.coverArts)
+        assertEquals(listOf("downloaded"), resolved?.downloadedCoverArts)
+    }
+
+    @Test
+    fun anOfflineArtistWithoutDownloadedCoversFallsBackToTheCreditCovers() {
+        val resolved =
+            resolveArtistArtInput(
+                ArtistArtInput(
+                    artist = artist(downloadedCoverArts = emptyList()),
+                    artistId = "ar1",
+                    offline = true,
+                    sourceId = 7,
+                    covers = listOf("a", "b"),
+                ),
+            )
+
+        assertEquals(listOf("a", "b"), resolved?.downloadedCoverArts)
+    }
+
+    @Test
+    fun anArtistWithMaterializedCoversIsKeptWhenTheCreditQueryIsEmpty() {
+        val original = artist(coverArts = listOf("materialized"))
+
+        val resolved =
+            resolveArtistArtInput(
+                ArtistArtInput(
+                    artist = original,
+                    artistId = "ar1",
+                    offline = false,
+                    sourceId = 7,
+                    covers = emptyList(),
+                ),
+            )
+
+        assertEquals(original, resolved)
+    }
+
+    @Test
+    fun noArtistAndNoCoversClearsTheArt() {
+        assertNull(
+            resolveArtistArtInput(
+                ArtistArtInput(
+                    artist = null,
+                    artistId = "ar1",
+                    offline = false,
+                    sourceId = 7,
+                    covers = emptyList(),
+                ),
+            ),
+        )
+    }
+
+    private fun artist(
+        coverArts: List<String> = emptyList(),
+        downloadedCoverArts: List<String> = emptyList(),
+    ) = Artist(
+        sourceId = 7,
+        id = "ar1",
+        name = "Artist",
+        albumCount = 1,
+        starred = null,
+        coverArts = coverArts,
+        downloadedCoverArts = downloadedCoverArts,
+    )
+}