feat(ui): move HSV swatch onto the SV handle

HsvColorPicker: read the color swatch from the SV handle's child
("Handle/Swatch") instead of a standalone root child; drop the dead
leftover-alpha bar and root-swatch fallback handling.
PaintHudPanel.prefab: restructure the picker so the swatch lives under
the SV handle and wire _swatch accordingly.

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

Assets/Scripts/Components/UI/HsvColorPicker.cs

@@ -66,6 +66,12 @@ public class HsvColorPicker : MonoBehaviour
 
             _svHandle = sv.Find("Handle") as RectTransform;
 
+            if (_svHandle)
+            {
+                var swatch = _svHandle.Find("Swatch");
+                if (swatch) _swatch = swatch.GetComponent<Image>();
+            }
+
             Rehook(sv, OnSvPicked);
         }
 
@@ -78,14 +84,6 @@ public class HsvColorPicker : MonoBehaviour
             Rehook(hue, OnHuePicked);
         }
 
-        // Alpha now lives on a standalone slider (PaintHud), not in the picker. Hide any alpha bar a
-        // pre-split prefab still carries so it does not show as a dead, non-functional strip.
-        var leftoverAlpha = transform.Find("Alpha");
-        if (leftoverAlpha) leftoverAlpha.gameObject.SetActive(false);
-
-        var swatch = transform.Find("Swatch");
-        if (swatch) _swatch = swatch.GetComponent<Image>();
-
         Refresh();
     }