[GUI] add game library watcher & remove rescan button (#678)

src/SharpEmu.GUI/GameLibraryReconciler.cs

@@ -0,0 +1,98 @@
+// Copyright (C) 2026 SharpEmu Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+namespace SharpEmu.GUI;
+
+internal sealed record GameLibraryReconciliation(
+    IReadOnlyList<GameEntry> Games,
+    IReadOnlyList<GameEntry> CoversToLoad,
+    IReadOnlySet<GameEntry> BackgroundsChanged);
+
+/// <summary>
+/// Applies filesystem scan results while preserving presentation object
+/// identity for games that remain in the library
+/// </summary>
+internal static class GameLibraryReconciler
+{
+    public static GameLibraryReconciliation Reconcile(
+        IReadOnlyList<GameEntry> current,
+        IReadOnlyList<GameEntry> scanned)
+    {
+        var existingByPath = current.ToDictionary(game => game.Path, GameLibraryPath.Comparer);
+        var merged = new List<GameEntry>(scanned.Count);
+        var coversToLoad = new List<GameEntry>();
+        var backgroundsChanged = new HashSet<GameEntry>();
+
+        foreach (var scannedGame in scanned)
+        {
+            if (!existingByPath.TryGetValue(scannedGame.Path, out var existing))
+            {
+                merged.Add(scannedGame);
+                if (scannedGame.CoverPath is not null)
+                {
+                    coversToLoad.Add(scannedGame);
+                }
+
+                continue;
+            }
+
+            var changes = existing.UpdateFrom(scannedGame);
+            merged.Add(existing);
+            if ((changes & GameEntryChanges.Cover) != 0 && existing.CoverPath is not null)
+            {
+                coversToLoad.Add(existing);
+            }
+
+            if ((changes & GameEntryChanges.Background) != 0)
+            {
+                backgroundsChanged.Add(existing);
+            }
+        }
+
+        return new GameLibraryReconciliation(merged, coversToLoad, backgroundsChanged);
+    }
+
+    /// <summary>
+    /// Reorders, inserts and removes only the items needed to reach the desired
+    /// visible sequence
+    /// </summary>
+    public static void ReconcileVisibleGames(
+        IList<GameEntry> visible,
+        IReadOnlyList<GameEntry> desired)
+    {
+        for (var index = 0; index < desired.Count; index++)
+        {
+            var game = desired[index];
+            if (index < visible.Count && ReferenceEquals(visible[index], game))
+            {
+                continue;
+            }
+
+            var existingIndex = -1;
+            for (var candidate = index + 1; candidate < visible.Count; candidate++)
+            {
+                if (ReferenceEquals(visible[candidate], game))
+                {
+                    existingIndex = candidate;
+                    break;
+                }
+            }
+
+            if (existingIndex >= 0)
+            {
+                var existing = visible[existingIndex];
+                visible.RemoveAt(existingIndex);
+                visible.Insert(index, existing);
+            }
+            else
+            {
+                visible.Insert(index, game);
+            }
+        }
+
+        while (visible.Count > desired.Count)
+        {
+            visible.RemoveAt(visible.Count - 1);
+        }
+    }
+}