Silence unchecked cast warning.

Technically, we had been assuming the root pointer's type was
Bosk<R>.RootRef, while it's actually only known to be
Bosk<?>.RootRef.

bosk-core/src/main/java/io/vena/bosk/Bosk.java

@@ -26,6 +26,7 @@
 import lombok.NonNull;
 import lombok.RequiredArgsConstructor;
 import lombok.Value;
+import lombok.val;
 import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 import org.slf4j.Logger;
@@ -289,7 +290,7 @@ private <T> void assertCorrectBosk(Reference<T> target) {
 			// On the other hand, there's a certain symmetry to requiring the references to have the right
 			// bosk for both reads and writes, and forcing this discipline on users might help them avoid
 			// some pretty confusing mistakes.
-			assert ((RootRef) target.root()).bosk() == Bosk.this: "Reference supplied to driver operation must refer to the correct bosk";
+			assert ((Bosk<?>.RootRef)target.root()).bosk() == Bosk.this: "Reference supplied to driver operation must refer to the correct bosk";
 		}
 
 		/**