feat: Side guard stub

nhf/touhou/shiki_eiki/crown.py

@@ -341,6 +341,16 @@ class Crown(Model):
             )
         return sketch.assemble()
 
+    def side_guard(self) -> Cq.Workplane:
+        rb = self.base_circ / (2 * math.pi)
+        rt = self.tilt_circ / (2 * math.pi)
+        outer = Cq.Solid.makeCone(
+            radius1=rb,
+            radius2=rt,
+            height=self.height,
+        )
+        return outer
+
     def assembly(self) -> Cq.Assembly:
         front = (
             Cq.Workplane('XY')