fix(objc): emit implements edge for protocol-to-protocol adoption

`@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.

graphify/extract.py

@@ -11625,6 +11625,18 @@ def extract_objc(path: Path) -> dict:
                 proto_nid = _make_id(stem, name)
                 add_node(proto_nid, f"<{name}>", line)
                 add_edge(file_nid, proto_nid, "contains", line)
+                # Adopted protocols: `@protocol Derived <Base, Other>`. These
+                # nest under a protocol_reference_list node (distinct from the
+                # parameterized_arguments node used by @interface adoption), so
+                # they were never emitted. Emit an `implements` edge for each,
+                # matching how @interface protocol adoption is handled.
+                for child in node.children:
+                    if child.type == "protocol_reference_list":
+                        for sub in child.children:
+                            if sub.type == "identifier":
+                                base_nid = ensure_named_node(_read(sub), line)
+                                if base_nid != proto_nid:
+                                    add_edge(proto_nid, base_nid, "implements", line)
                 for child in node.children:
                     walk(child, proto_nid)
             return

tests/fixtures/sample.m

@@ -40,3 +40,15 @@
 }
 
 @end
+
+@protocol Base
+
+- (void)baseMethod;
+
+@end
+
+@protocol Derived <Base>
+
+- (void)derivedMethod;
+
+@end

tests/test_languages.py

@@ -1064,6 +1064,16 @@ def test_objc_splits_inherits_and_implements():
     assert ("Animal", "SampleDelegate") in _edge_labels(r, "implements")
 
 
+def test_objc_protocol_adopts_protocol():
+    """`@protocol Derived <Base>` must emit an implements edge Derived->Base.
+    Protocol-on-protocol adoption nests under a protocol_reference_list node
+    (distinct from the parameterized_arguments node used by @interface
+    adoption), so the edge was previously dropped. Protocol nodes are labeled
+    `<Name>`, so the edge reads (<Derived>, <Base>)."""
+    r = extract_objc(FIXTURES / "sample.m")
+    assert ("<Derived>", "<Base>") in _edge_labels(r, "implements")
+
+
 def test_objc_property_type_context():
     r = extract_objc(FIXTURES / "sample.m")
     assert ("Animal", "NSString") in _edge_labels(r, "references", "field")