disallow running another reticle after a dotter

crates/haku/src/trampoline.rs

@@ -73,6 +73,12 @@ impl Trampoline {
         let value = vm.run(system, draw_id, &[Value::Ref(dotter)])?;
         self.value = value;
 
+        // Prevent event handling weirdness on the frontend by disallowing continuing dotters
+        // with anything but scribbles.
+        if self.cont(vm) != Cont::Scribble {
+            return Err(vm.create_exception("a dotter must be continued with a scribble"));
+        }
+
         Ok(())
     }
 }