fix(sketch): a centerline can be switched straight to construction

sketch.construction set `construction` but left `centerline` on, so the
curve stayed a centerline (drawn dash-dot, still marked as an axis) until
it was first switched back to normal. Making a curve construction now
clears its centerline flag, mirroring sketch.centerline.

crates/engine/src/cmd/sketch.rs

@@ -1703,14 +1703,18 @@ fn construction(s: &mut Session, p: &Value) -> Result<Value> {
         let cs: Vec<usize> =
             ids.iter().map(|id| sk.curve_index(id).ok_or_else(|| bad(cmd, format!("unknown curve `{id}`")))).collect::<Result<_>>()?;
         // The selection switches together (a slot or polygon as a whole): to construction
-        // unless all of it already is. Projected curves may switch too.
+        // unless all of it already is. Projected curves may switch too. A centerline that
+        // becomes construction stops being a centerline (one line type at a time).
         let to = want.unwrap_or_else(|| !cs.iter().all(|c| sk.curves.get(*c).is_some_and(|c| c.construction)));
         let mut n = 0;
         for c in cs {
             if let Some(cu) = sk.curves.get_mut(c)
-                && cu.construction != to
+                && (cu.construction != to || to && cu.centerline)
             {
                 cu.construction = to;
+                if to {
+                    cu.centerline = false;
+                }
                 n += 1;
             }
         }