feat(android): add HomeArtistPrewarmer (top-8 artists, session-deduped)

android/app/src/main/java/com/fabledsword/minstrel/metadata/HomeArtistPrewarmer.kt

@@ -0,0 +1,33 @@
+package com.fabledsword.minstrel.metadata
+
+import java.util.Collections
+import javax.inject.Inject
+import javax.inject.Singleton
+
+private const val TOP_N = 8
+
+/**
+ * Pre-warms the Room cache for likely-tap artist tiles on Home. Mirrors
+ * Flutter's `MetadataPrefetcher`: conservative — artists only (a single
+ * round-trip per id), top-8 per call, and once per id per session so
+ * repeated index refreshes don't re-fetch. Albums are intentionally not
+ * pre-warmed (their detail fetch fans out a track list).
+ *
+ * Fetches go through [MetadataProvider.refreshArtist], so they share the
+ * same per-id dedup AND the 4-slot concurrency gate as on-render
+ * hydration — the prewarm can't starve the tiles the user is looking at.
+ */
+@Singleton
+class HomeArtistPrewarmer @Inject constructor(
+    private val metadataProvider: MetadataProvider,
+) {
+    private val warmed = Collections.synchronizedSet(mutableSetOf<String>())
+
+    fun warm(artistIds: List<String>) {
+        artistIds.asSequence()
+            .filter { it.isNotEmpty() }
+            .filter { warmed.add(it) }
+            .take(TOP_N)
+            .forEach { metadataProvider.refreshArtist(it) }
+    }
+}

android/app/src/test/java/com/fabledsword/minstrel/metadata/HomeArtistPrewarmerTest.kt

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+package com.fabledsword.minstrel.metadata
+
+import io.mockk.every
+import io.mockk.mockk
+import io.mockk.verify
+import org.junit.jupiter.api.Test
+
+class HomeArtistPrewarmerTest {
+    private val provider = mockk<MetadataProvider>()
+
+    @Test
+    fun `warms at most the top eight distinct ids, once per session`() {
+        every { provider.refreshArtist(any()) } returns mockk(relaxed = true)
+        val prewarmer = HomeArtistPrewarmer(provider)
+
+        // 10 ids with a duplicate and a blank -> 8 distinct non-blank warmed.
+        prewarmer.warm(listOf("a1", "a2", "a3", "a4", "a5", "a6", "a7", "a8", "a9", ""))
+        // Re-warm: already-warmed ids are skipped.
+        prewarmer.warm(listOf("a1", "a2"))
+
+        (1..8).forEach { i -> verify(exactly = 1) { provider.refreshArtist("a$i") } }
+        verify(exactly = 0) { provider.refreshArtist("a9") }   // past the top-8 cap
+        verify(exactly = 0) { provider.refreshArtist("") }     // blank skipped
+    }
+}