Fix subscriptions for read only signals passed as props (#3173)

* add a fuzzing test for reference counted generational boxes

* Fix subscriptions for readonlysignal passed as props

* Don't subscribe to dropped contexts

---------

Co-authored-by: Jonathan Kelley <jkelleyrtp@gmail.com>

packages/signals/src/read_only_signal.rs

@@ -43,7 +43,7 @@ impl<T: 'static, S: Storage<SignalData<T>>> ReadOnlySignal<T, S> {
     }
 
     /// Point to another signal
-    pub fn point_to(&mut self, other: Self) -> BorrowResult {
+    pub fn point_to(&self, other: Self) -> BorrowResult {
         self.inner.point_to(other.inner)
     }