Simplify HUD layout save path to a fixed PaintHud.json

Drop the scene-derived hudSaveId/key mechanism and save the HUD layout to a
single PaintHud.json under persistentDataPath.

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

Assets/Scripts/Components/UI/PaintHud/PaintHud.Persistence.cs

@@ -20,10 +20,6 @@ public partial class PaintHud
              "defaults every session.")]
     public bool persistHud = true;
 
-    [Tooltip("Save file identifier. Empty = derived from the HUD's own scene name. Set explicitly to keep " +
-             "the layout stable across scene renames or to share one layout between scenes.")]
-    public string hudSaveId = "";
-
     private const int HudFileVersion = 1;
 
     // Auto-save bookkeeping. Brush edits are debounced (sliders fire every frame while dragged): the due
@@ -60,21 +56,11 @@ public partial class PaintHud
     private bool _hasDefaults;    // non-camo defaults captured (first bind, before restore/carry)
     private bool _hasDefaultCamo; // camo defaults captured (first ready-gate, before any pending camo apply)
 
-    private static string HudSaveDir => Path.Combine(Application.persistentDataPath, "MeshPaint");
-
     private string HudSavePath()
     {
-        // Derive from the HUD's OWN scene (a stable persistent scene), not the active scene: the latter
-        // now changes across the additive lobby/map loads, so a key from it would split save and restore
-        // into different files and silently lose the saved layout.
-        string key = string.IsNullOrEmpty(hudSaveId)
-            ? SanitizeHudKey(gameObject.scene.name + "_hud")
-            : SanitizeHudKey(hudSaveId);
-        return Path.Combine(HudSaveDir, key + ".hudlayout");
+        return Path.Combine(Application.persistentDataPath, "PaintHud.json");
     }
 
-    private static string SanitizeHudKey(string s) => MeshPainter.SanitizeFileKey(s);
-
     [Serializable]
     private class HudState
     {
@@ -145,7 +131,7 @@ public partial class PaintHud
                 state.hasHeightValues = _camoEverReady;
             }
 
-            Directory.CreateDirectory(HudSaveDir);
+            Directory.CreateDirectory(Application.persistentDataPath);
             File.WriteAllText(HudSavePath(), JsonUtility.ToJson(state, true));
 
             // Refresh the on-disk baseline so the debounce does not immediately re-save unchanged values.