[jvm] fix self-recursing Object bridge for closures with non-Object typed return

[kLabz]

When a closure's typed return is a specific reference type (e.g. SyncStats) and a functional interface in the conversion set has an Object-erased abstract method also named invoke — the shape of Function0, Callable after erasure — the FI loop in jvmFunctions.generate_invoke synthesized an Object invoke() bridge but pointed it at (meth.name, meth.dargs, meth.dret). At that program point meth came from register_signature with dret already declassified to Object, so the bridge body emitted invokevirtual invoke()Object against itself. The later loop meth_typed block that would have produced the proper Object→typed bridge was then short-circuited by has_method (the slot was already taken). First invocation StackOverflowError'd.

Forward to (meth.name, arg_sigs, ret) — the typed invoke that was actually spawned earlier in generate_invoke — so the Object-returning bridge dispatches to the real implementation and casts the result on return.

Repro shape (also added as a misc/jvm test extending StructuralSam):

  public interface GenericInvoke<T> { T invoke(); }
  Listeners.runGenericInvoke(() -> new Boxed(7));