fix(view): apply saved view twist as camera roll on load

Drawings authored under a rotated UCS save a view twist so they open
square. camera_from_vport ignored VPort.view_twist (roll hardcoded 0),
so such files rendered in raw world orientation and looked tilted.

Feed -view_twist in as the camera roll: the twist rotates world-X onto
screen-right, so the direction that becomes horizontal is its negative.
The roll rides the camera quaternion, surviving render and orbit.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/scene/mod.rs

@@ -6131,7 +6131,13 @@ impl Scene {
         } else {
             vd.x.atan2(-vd.y)
         };
-        let rotation = view::camera::yaw_pitch_to_quat(yaw, pitch, 0.0);
+        // The saved view can carry a twist (rotation about the view axis), set
+        // when the view was aligned to a rotated UCS. The twist is the angle
+        // that rotates world-X onto screen-right, so the world direction that
+        // ends up horizontal is its negative; feed that in as the camera roll
+        // so the drawing opens square, the way it was saved, instead of in raw
+        // world orientation (which looks tilted).
+        let rotation = view::camera::yaw_pitch_to_quat(yaw, pitch, -vp.view_twist as f32);
         let view_right = rotation * glam::Vec3::X;
         let view_up = rotation * glam::Vec3::Y;
         // view_target is WCS; wire-space subtracts world_offset.