fix(rust): emit field type references for tuple structs

extract_rust() only traversed field_declaration_list (named-struct
bodies), so tuple structs -- whose positional fields nest under
ordered_field_declaration_list -- had every field type reference
silently dropped from the graph.

This is the same node shape the enum handler already accounts for
(tuple variants nest their types under ordered_field_declaration_list);
the struct path was simply left behind. Add an additive branch that,
for each type node in a tuple struct's ordered_field_declaration_list,
collects type refs via _rust_collect_type_refs and emits references
edges with the appropriate field / generic_arg context. The
named-struct path is untouched.

For `struct Wrapper(Logger, Config);` with Logger/Config defined
in-file, no field edges were produced before; both are now emitted.

Adds test_rust_tuple_struct_field_references and a tuple struct to the
shared Rust fixture covering plain and generic positional field types.

tests/fixtures/sample.rs

@@ -56,3 +56,5 @@ enum GraphEvent {
     NodeAdded(Graph),
     Processed { proc: DataProcessor },
 }
+
+struct GraphPair(Graph, Result<DataProcessor>);

tests/test_multilang.py

@@ -365,6 +365,19 @@ def test_rust_struct_field_emits_field_context():
     assert ("DataProcessor", "DataProcessor") not in _edge_labels(r, "references", "field")
 
 
+def test_rust_tuple_struct_field_references():
+    """Tuple struct fields (`struct Wrapper(A, B);`) nest their positional types
+    under ordered_field_declaration_list with no field_declaration wrapper -- the
+    same shape as tuple enum variants. That path was not traversed for structs, so
+    tuple-struct field type references were silently dropped.
+    """
+    r = extract_rust(FIXTURES / "sample.rs")
+    field_refs = _edge_labels(r, "references", "field")
+    assert ("GraphPair", "Graph") in field_refs, "tuple-struct field reference missing"
+    assert ("GraphPair", "Result") in field_refs, "tuple-struct generic field reference missing"
+    assert ("GraphPair", "DataProcessor") in _edge_labels(r, "references", "generic_arg")
+
+
 def test_rust_method_parameter_return_and_generic_contexts():
     r = extract_rust(FIXTURES / "sample.rs")
     assert ("build", "DataProcessor") in _edge_labels(r, "references", "parameter_type")