feat(cad): add the 3dfiles package and the original drive shaft mount

cad/.envrc

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+use flake ..#cad

cad/build.py

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+# pyright: reportUnusedCallResult=false, reportWildcardImportFromLibrary=false
+from build123d import *
+
+hole_offset = 20
+hole_diameter = 22
+
+width = 40
+thickness = 20
+
+assert (hole_offset - hole_diameter / 2) > 0
+assert (width - hole_diameter) > 0
+
+
+with BuildPart() as builder:
+    with BuildSketch() as sketch:
+        with BuildLine() as profile:
+            l1 = Polyline(
+                (0,0),
+                (width / 2, 0),
+                (width / 2, hole_offset)
+            )
+            l2 = CenterArc((0, hole_offset), width / 2, 90, -90)
+            mirror(profile.line, Plane.YZ)
+        make_face()
+
+        with Locations((0, hole_offset)):
+            Circle(hole_diameter / 2, mode=Mode.SUBTRACT)
+
+    extrude(amount=thickness / 2)
+    mirror(about=Plane.XY)
+
+assert builder.part is not None
+export_step(builder.part, "drive shaft mount.step")