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/copy_value.rs

@@ -80,7 +80,7 @@ impl<T: 'static, S: Storage<T>> CopyValue<T, S> {
     }
 
     /// Point to another copy value
-    pub fn point_to(&mut self, other: Self) -> BorrowResult {
+    pub fn point_to(&self, other: Self) -> BorrowResult {
         self.value.point_to(other.value)
     }