feat: add line-foot construction method (#48)

With this PR, the adapptlet appears to handle Joyce's entire Euclid Book I.

Reviewed-on: #48
Co-authored-by: Glen Whitney <glen@studioinfinity.org>
Co-committed-by: Glen Whitney <glen@studioinfinity.org>

src/adapptlet.civet

@@ -728,6 +728,17 @@ classHandler: Record<JoyceClass, ClassHandler> :=
             commands.push `${aux} = Translate(${source}, ${displacement})`
             auxiliaries.push aux
             ends[1] = aux
+         'foot'
+            pt := args.subpoints
+            unless pt then return
+            destination := args.plane
+               ? args.plane[0]
+               : `Line(${pt[1]},${pt[2]})`
+            ends[0] = pt[0]
+            commands.push
+               `${aux} = ClosestPoint(${destination}, ${pt[0]})`
+            auxiliaries.push aux
+            ends[1] = aux
          'parallel'
             unless args.subpoints then return
             [newStart, oldStart, oldEnd] := args.subpoints