Fix USD camera tumble: lossless matrix write-back + gimbal-free orbit init

Two issues caused the camera orientation to flip when tumbling a custom
USD camera:

1. Write-back decomposed the camera matrix to XYZ-euler via
   Decompose(X,Y,Z) and re-authored it as a rotateXYZ op. That round-trip
   is lossy — the angles Decompose returns do not reconstruct the same
   matrix as a rotateXYZ op, so the orientation read back from the prim
   differed from the free-camera view shown during the drag. The view
   jumped every time a drag finished and snapped back on the next drag.
   Now author the full camera-to-world transform as a single matrix op,
   which round-trips exactly through UsdGeomCamera::GetCamera().

2. Orbit init relied on PullFromCameraTransform's Euler decomposition for
   theta/phi, which is gimbal-affected. Added
   ViewportCamera::InitOrbitFromEyeAndCenter to derive theta/phi directly
   from the eye->center vector (zero roll), respecting the Z-up matrix.

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

src/core/ViewportCamera.h

@@ -121,6 +121,14 @@ public:
         m_cameraTransformDirty = true;
     }
 
+    /// Set orbital state from eye position and orbit center without Euler decomposition.
+    /// Computes theta/phi directly from the eye→center vector in the Y-up orbital frame;
+    /// zeroes roll.  Use this after SwitchToFreeCamera() when initialising from a USD
+    /// camera prim to avoid gimbal-lock artifacts from the Decompose() path.
+    void InitOrbitFromEyeAndCenter(const pxr::GfVec3d& eye,
+                                   const pxr::GfVec3d& center,
+                                   double dist);
+
     /// Returns the current near-clip distance.
     double GetNearClip() const {
         return static_cast<double>(m_camera.GetClippingRange().GetMin());