feat(canvas): add artwork selection store

Implement an in-memory store for canvas artwork selections per track.

Author: wynriu

app/src/main/kotlin/com/metrolist/music/features/canvas/CanvasArtworkSelectionStore.kt

@@ -0,0 +1,22 @@
+package com.metrolist.music.features.canvas
+
+import kotlinx.coroutines.flow.MutableStateFlow
+import kotlinx.coroutines.flow.StateFlow
+import kotlinx.coroutines.flow.asStateFlow
+import kotlinx.coroutines.flow.update
+import java.util.concurrent.ConcurrentHashMap
+
+/** In-memory per-track selections made from the Player's Canvas picker. */
+object CanvasArtworkSelectionStore {
+    private val selected = ConcurrentHashMap<String, CanvasArtwork>()
+    private val _changes = MutableStateFlow(0L)
+    val changes: StateFlow<Long> = _changes.asStateFlow()
+
+    fun get(mediaId: String): CanvasArtwork? = selected[mediaId]
+
+    fun put(mediaId: String, artwork: CanvasArtwork) {
+        if (mediaId.isBlank()) return
+        selected[mediaId] = artwork
+        _changes.update { it + 1L }
+    }
+}