fix(viewcube): deterministic horizon for obliques, generic opposite

Top Front Right/Left kept Y (north) as up when starting from Top,
tilting the base because orbit is turntable (never banks). Make
snap_to_direction deterministic: world +Z horizon except near +-Z
where north +Y is used, sign-preserved only for intentionally
inverted views. Expand opposite() from faces-only to all 26 regions
so already-there flip works for edges/corners.

src/scene/pipeline/viewcube.rs

@@ -142,24 +142,39 @@ impl CubeRegion {
 
     /// Unit eye-direction vector (from target toward the camera) that
     /// looks straight at this region. Used by `Camera::snap_to_direction`
-    /// which derives the full orientation by re-using the current
-    /// camera's up vector, projected onto the plane perpendicular to
-    /// this direction — so clicking an edge spins the cube around the
-    /// edge without rolling the user's "up" sense.
+    /// / `snap_to_face` which derive a deterministic horizon (world +Z,
+    /// except top/bottom where north +Y is used) so every cube click
+    /// lands repeatably.
     pub fn snap_direction(self) -> glam::Vec3 {
         let c = region_centroids()[self.id()];
         glam::Vec3::new(c[0], c[1], c[2]).normalize_or(glam::Vec3::Z)
     }
 
     pub fn opposite(self) -> CubeRegion {
-        match self {
-            CubeRegion::Face(FACE_TOP) => CubeRegion::Face(FACE_BOTTOM),
-            CubeRegion::Face(FACE_BOTTOM) => CubeRegion::Face(FACE_TOP),
-            CubeRegion::Face(FACE_FRONT) => CubeRegion::Face(FACE_BACK),
-            CubeRegion::Face(FACE_BACK) => CubeRegion::Face(FACE_FRONT),
-            CubeRegion::Face(FACE_RIGHT) => CubeRegion::Face(FACE_LEFT),
-            CubeRegion::Face(FACE_LEFT) => CubeRegion::Face(FACE_RIGHT),
-            other => other,
+        // Generic opposite: the region whose centroid is nearest to -self.
+        // Covers faces, edges and corners so "already there → flip" works
+        // for every cube click, not just faces.
+        let centroids = region_centroids();
+        let c = centroids[self.id()];
+        let neg = [-c[0], -c[1], -c[2]];
+        let mut best = self.id();
+        let mut best_d = f32::MAX;
+        for (i, cc) in centroids.iter().enumerate() {
+            let dx = cc[0] - neg[0];
+            let dy = cc[1] - neg[1];
+            let dz = cc[2] - neg[2];
+            let d = dx * dx + dy * dy + dz * dz;
+            if d < best_d {
+                best_d = d;
+                best = i;
+            }
+        }
+        if best < 6 {
+            CubeRegion::Face(best)
+        } else if best < 18 {
+            CubeRegion::Edge(best)
+        } else {
+            CubeRegion::Corner(best)
         }
     }