`@protocol Derived <Base>` dropped the protocol-adoption (inheritance)
edge. The protocol_declaration handler in extract_objc walked children
for method declarations but ignored the protocol_reference_list child
that holds the adopted protocols, so no implements edge was ever emitted
for protocol-on-protocol adoption.
The extractor already handled `@interface Foo <Proto>` adoption, but that
nests the protocol name under a parameterized_arguments node; protocol-on-
protocol adoption uses a different grammar node (protocol_reference_list)
whose adopted-name is a direct `identifier` child, so it was never
matched. Walk protocol_reference_list and emit an implements edge for each
adopted protocol, mirroring the @interface handling.
Adds a defined Base/Derived protocol pair to the ObjC fixture and a
regression test asserting the Derived->Base implements edge.
Synvoya