SignalRegistration

Registers signals on a registered class and emits them.

Registration uses classdb_register_extension_class_signal. Emission goes through Object.emit_signal, which is a vararg method, so it uses the Variant call path (object_method_bind_call) with the signal name as the first Variant argument followed by the signal's payload Variants.

This first cut covers the most common case — a no-argument signal — which is enough to prove registration + connect + emit end to end. Adding typed payload arguments is a matter of extending the argument-info array and the emit arg list.