C extraction: a macro-argument identifier becomes a USAGE edge to a same-named struct field #2666

C extraction turns a macro-argument identifier into an edge to an unrelated struct field of the same name.

tests/test_main.c runs every suite through a macro, e.g. RUN_SELECTED_SUITE(mcp);. The argument mcp is a suite name, but the graph records a USAGE edge from main to whichever Field is named mcp: in one index openhands_fixture_t.mcp (char mcp[640] in tests/test_cli.c), in another cbm_daemon_application_session.mcp (src/daemon/application.c).

A data member that is only named as a macro argument is not used there, so the edge is wrong for every consumer (trace_path, impact, detect_changes, test-impact selection).

Repro: index this repository, then

query_graph: MATCH (m)-[r]->(f) WHERE m.name = 'main' AND m.file_path = 'tests/test_main.c' AND f.name = 'mcp' RETURN type(r), labels(f), f.qualified_name

returns USAGE, ["Field"], a struct member.

Expected: an identifier that appears only as a macro argument resolves to nothing, or at least never to a Field of an unrelated struct.

Found while making PR test selection narrow: the test-impact engine reached the runner's main through this edge and ran every test. The Smart CI PR guards the engine against it (an edge from the runner's main into a Field named like one of its suites no longer counts); this issue is the extractor fix itself.