Move from class-scoped to instance-scoped InvocationHandler in generated proxies:

• Define a private field $mmHandler on the proxy class and route calls via InvocationHandlerAdapter.toField("$mmHandler") instead of InvocationHandlerAdapter.of(...).
• After creating the proxy instance (Objenesis), set the field reflectively.

This prevents the proxy class from holding a class-level reference to the handler, which previously retained ExplicitMappingBuilder and InheritingConfiguration captured by the handler.
In issue #375-like scenarios, this reduces retained heap and avoids long-lived retention without changing behavior.