Add Shift brush controls to paint mode

Hold Shift to momentarily enable the pipette (restores prior state on
release) and Shift+Scroll to resize the brush. The orbit zoom ignores the
wheel while Shift is held so it does not zoom and resize at once. The HUD
brush-size slider follows live via a BrushSizeChanged event, and the brush
preview now stays visible while Shift is held (drawn before the pipette
early-return) so the new size is visible while scrubbing.

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

Assets/Scripts/ThirdPersonOrbitInput.cs

@@ -133,7 +133,9 @@ public class ThirdPersonOrbitInput : MonoBehaviour
                 _zoomInit = true;
             }
 
-            float scroll = ReadScroll();
+            // Shift+wheel is reserved for the paint brush resize (MeshPainter), so the orbit camera
+            // ignores the wheel while Shift is held - otherwise it would zoom and resize at once.
+            float scroll = ShiftHeld() ? 0f : ReadScroll();
             if (!Mathf.Approximately(scroll, 0f))
             {
                 // One wheel notch = one zoom step (sign only), independent of the
@@ -199,6 +201,19 @@ public class ThirdPersonOrbitInput : MonoBehaviour
 #endif
     }
 
+    static bool ShiftHeld()
+    {
+#if ENABLE_INPUT_SYSTEM
+        var kb = Keyboard.current;
+        if (kb != null) return kb.leftShiftKey.isPressed || kb.rightShiftKey.isPressed;
+#endif
+#if ENABLE_LEGACY_INPUT_MANAGER
+        return Input.GetKey(KeyCode.LeftShift) || Input.GetKey(KeyCode.RightShift);
+#else
+        return false;
+#endif
+    }
+
     Vector2 ReadMouseDelta()
     {
 #if ENABLE_INPUT_SYSTEM