Fix ElevationDecorator arrow direction in drawing camera views

The elevation tag's local -Z axis is intentionally parallel to its
drawing camera's view direction, making screen-space projection of that
axis always degenerate (zero XY delta). Fall through to the tag's local
+X axis, which lies in the camera plane and rotates correctly as the
user adjusts the tag's orientation. Also fix a zero-length vector crash
in svgwriter when the same degenerate case occurs during SVG export.

src/bonsai/bonsai/bim/module/drawing/decoration.py

@@ -1522,6 +1522,20 @@ class ElevationDecorator(BaseDecorator):
             "output_edges": output_edges,
         }
 
+        # Determine the arrow direction in camera-image-plane (XY) space.
+        # The elevation tag's local -Z is intentionally parallel to the drawing
+        # camera's view direction, so projecting it always gives a near-zero XY
+        # delta.  Fall through to local +X (which is perpendicular to the view
+        # and rotates visibly when the user spins the tag).
+        view_mat = context.region_data.view_matrix
+        edge_dir_2d = Vector((1.0, 0.0))  # final fallback
+        for local_axis in (Vector((0, 0, -1)), Vector((1, 0, 0)), Vector((0, 1, 0))):
+            world_axis = obj.matrix_world.to_3x3() @ local_axis
+            cam_xy = (view_mat.to_3x3() @ world_axis).xy
+            if cam_xy.length > 1e-6:
+                edge_dir_2d = cam_xy.normalized()
+                break
+
         # process edges
         for edge in edges_original:
             v0, v1 = winspace_verts[edge[0]], winspace_verts[edge[1]]
@@ -1530,7 +1544,7 @@ class ElevationDecorator(BaseDecorator):
             circle_head = get_circle_head(circle_size)
             start_i = add_verts_sequence(add_offsets(v0, circle_head), start_i, **out_kwargs, closed=True)
 
-            edge_dir = (v1 - v0).normalized()
+            edge_dir = edge_dir_2d.to_3d()
             side = (edge_dir.yx * Vector((1, -1))).to_3d()
             triangle_head = get_triangle_head(side, edge_dir, triangle_length, triangle_width)
             start_i = add_verts_sequence(add_offsets(v0, triangle_head), start_i, **out_kwargs, closed=True)

src/bonsai/bonsai/bim/module/drawing/svgwriter.py

@@ -872,7 +872,11 @@ class SvgWriter:
 
         v1 = self.project_point_onto_camera(obj.matrix_world @ Vector((0, 0, 0)))
         v2 = self.project_point_onto_camera(obj.matrix_world @ Vector((0, 0, -1)))
-        angle = -math.degrees((v2 - v1).xy.angle_signed(Vector((0, 1))))
+        delta = (v2 - v1).xy
+        if delta.length <= 1e-6:
+            v2 = self.project_point_onto_camera(obj.matrix_world @ Vector((1, 0, 0)))
+            delta = (v2 - v1).xy
+        angle = -math.degrees(delta.angle_signed(Vector((0, 1)))) if delta.length > 1e-6 else 90.0
 
         transform = "rotate({}, {}, {})".format(angle, *symbol_position_svg.xy)