Fix residual roll inversion: CV->GL camera conversion is 180 about local Y, not X

public/js/exhibit.js

@@ -16,12 +16,13 @@ const $ = (s) => document.querySelector(s);
 
 /* The fused pose is a camera-in-world transform in OpenCV convention
  * (+X right, +Y DOWN, +Z FORWARD along the view axis). A Three.js camera uses
- * OpenGL convention (+X right, +Y UP, looks down LOCAL -Z). The two differ by a
- * 180° rotation about the camera's local X axis. Without this the world renders
- * upside-down and back-to-front (camera appears inverted) even though the pose
- * position and scene orientation are correct. Applied after copying the fused
- * quaternion each frame. */
-const CV_TO_GL = new THREE.Quaternion().setFromAxisAngle(new THREE.Vector3(1, 0, 0), Math.PI);
+ * OpenGL convention (+X right, +Y UP, looks down LOCAL -Z). The correct change of
+ * basis between them is a 180° rotation about the camera's local Y axis: it flips
+ * the forward axis (+Z_cv view -> three.js -Z) AND the up axis (+Y_cv down ->
+ * three.js +Y up) together, leaving roll correct. (A 180° about X fixes the view
+ * axis too but leaves roll inverted — ghosts render upside-down when the phone is
+ * upright.) Applied after copying the fused quaternion each frame. */
+const CV_TO_GL = new THREE.Quaternion().setFromAxisAngle(new THREE.Vector3(0, 1, 0), Math.PI);
 
 let info = { mode: 'dev' };
 let scene3, camera3, renderer, video, canvas2d, ctx2d;