feat: Implement 'similar' point method for Steve Gray problem

src/adapptlet.civet

@@ -500,9 +500,26 @@ classHandler: Record<JoyceClass, ClassHandler> :=
             [center, direction] := args.subpoints
             // Note clockwise 90° rotation (3π/2) confirmed in Joyce source
             commands.push `${name} = Rotate(${direction}, 3*pi/2, ${center})`
-         'proportion'
+         /proportion|similar/
-            pt := args.subpoints
+            pt .= args.subpoints
             unless pt then return
+            // reduce the similar case to general proportion
+            if method is 'similar'
+               unless pt.length is 5 then return
+               sourcePlane .= ''
+               destPlane .= ''
+               if is3d
+                  unless args.plane then return
+                  destPlane = `, ${args.plane[0]}`
+                  if args.plane.length > 1
+                     sourcePlane = `, ${args.plane[1]}`
+                  else
+                     sourcePlane = `, Plane(${pt[2]}, ${pt[3]}, ${pt[4]})`
+               angle := `Angle(${pt[3]}, ${pt[2]}, ${pt[4]}${sourcePlane})`
+               commands.push
+                  `${aux} = Rotate(${pt[1]}, ${angle}, ${pt[0]}${destPlane})`
+               auxiliaries.push aux
+               pt = [pt[2], pt[3], pt[2], pt[4], pt[0], pt[1], pt[0], aux]
             len := `Distance(${pt[2]},${pt[3]})*Distance(${pt[4]},${pt[5]})`
                + `/ Distance(${pt[0]},${pt[1]})`
             direction := `UnitVector(Vector(${pt[6]}, ${pt[7]}))`