Fix spurious X/Y rotation on fillet corner wall

When the two source walls were placed at different elevations, the
fillet corner wall ended up with sub-degree X and Y Euler rotations
even though both source walls had only a Z rotation.

Cause: _apply_fillet_corner_geometry derived the corner's local X
axis from `chord = tangent_b - tangent_a` (a 3D vector). With walls
at different Z, `chord.z` was non-zero, so `x_dir = chord.normalized()`
inherited that Z component. The Z axis was already hardcoded to world
Z, so x_dir and z_dir were no longer orthogonal — the resulting
matrix_world was non-orthonormal, and Blender's Euler decomposition
surfaced the skew as the visible X/Y rotation drift.

Project the chord to the XY plane before normalising so x_dir is
strictly XY-aligned and orthogonal to z_dir. The corner wall is now
placed at wall A's elevation with a pure Z rotation, which matches
the user's expectation when both inputs are Z-aligned regardless of
their relative elevation.

Generated with the assistance of an AI coding tool.

src/bonsai/bonsai/bim/module/model/wall.py

@@ -2651,7 +2651,17 @@ def _apply_fillet_corner_geometry(
     if body_context is None:
         return None
 
-    x_dir = chord.normalized()
+    # Project the chord to the XY plane for the local-frame X axis. The
+    # corner wall's Z axis is hardcoded to world Z below, so an XY-aligned
+    # X axis is required for an orthonormal rotation matrix. Without the
+    # projection, any chord Z component (walls placed at different
+    # elevations) leaves x_dir non-orthogonal to z_dir and Blender's
+    # Euler decomposition surfaces the skew as spurious sub-degree X/Y
+    # rotations on the corner.
+    chord_xy = Vector((chord.x, chord.y, 0.0))
+    if chord_xy.length < 1e-6:
+        return None
+    x_dir = chord_xy.normalized()
     z_dir = Vector((0.0, 0.0, 1.0))
     y_dir = z_dir.cross(x_dir).normalized()
     corner_obj.matrix_world = Matrix(