Fix paint HUD settings not persisting until a panel is dragged

The debounced auto-save compared painter values against the on-disk
baseline, so after any slider/toggle change the values stayed different
every frame and the 0.5s timer was pushed forward endlessly, never
firing. Saving only happened via the direct drag-end path. Compare
against the previous-frame snapshot instead so the timer fires once the
values settle. Also pin hudSaveId to a stable key so the layout is
shared across scenes instead of split per-scene.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Scenes/Main.unity

@@ -2057,7 +2057,7 @@ MonoBehaviour:
   m_Name: 
   m_EditorClassIdentifier: Assembly-CSharp::PaintHud
   persistHud: 1
-  hudSaveId: 
+  hudSaveId: PaintHUD
   modeManager: {fileID: 0}
   painter: {fileID: 0}
   layoutPrefab: {fileID: 3563111358790320208, guid: fa9dc2ea9e2c9ac4f815f666014804c8, type: 3}

Assets/Scripts/PaintHud.Persistence.cs

@@ -39,7 +39,8 @@ public partial class PaintHud
     // fire every frame while dragged - we push the due time forward and write once they settle. No
     // teardown save, so persistence never depends on a clean quit.
     private const float HudSaveDebounce = 0.5f;
-    private readonly HudValues _savedValues = new HudValues(); // mirror of what is currently on disk
+    private readonly HudValues _savedValues = new HudValues();    // mirror of what is currently on disk
+    private readonly HudValues _lastSeenValues = new HudValues(); // painter values observed last frame (settle detection)
     private bool _hasSavedBaseline;
     private bool _hudSavePending;
     private float _hudSaveDueTime;
@@ -183,12 +184,20 @@ public partial class PaintHud
             // so it is immune to the destroy-pending timing of the despawn.
             CaptureCarry();
 
-            // A brush-value edit differs from what is on disk -> schedule a coalesced save. Sliders fire
-            // every frame while dragged, so we keep pushing the due time forward and write once they settle.
-            if (persistHud && _hasSavedBaseline && !ValuesEqual(painter, _savedValues))
+            // Debounced auto-save: (re)arm the settle timer only on frames where a value actually CHANGED
+            // since the last frame - not merely differs from disk. Sliders fire every frame while dragged,
+            // so this keeps the deadline ahead of the drag and writes once the values stop moving. Comparing
+            // against the on-disk baseline instead would push the deadline forward forever (the edit always
+            // differs from disk until it is written), so the save would never fire on its own - it only
+            // landed when something else, e.g. a panel drop, wrote the state directly.
+            if (persistHud && _hasSavedBaseline && !ValuesEqual(painter, _lastSeenValues))
             {
-                _hudSavePending = true;
-                _hudSaveDueTime = Time.unscaledTime + HudSaveDebounce;
+                CopyValues(painter, _lastSeenValues);
+                if (!ValuesEqual(painter, _savedValues))
+                {
+                    _hudSavePending = true;
+                    _hudSaveDueTime = Time.unscaledTime + HudSaveDebounce;
+                }
             }
         }
 
@@ -199,7 +208,11 @@ public partial class PaintHud
     /// <summary>Marks the current brush values as the on-disk baseline so auto-save only fires on real edits.</summary>
     private void SnapshotSavedBaseline()
     {
-        if (painter) CopyValues(painter, _savedValues);
+        if (painter)
+        {
+            CopyValues(painter, _savedValues);
+            CopyValues(painter, _lastSeenValues);
+        }
         _hasSavedBaseline = painter;
         _hudSavePending = false;
     }