build: api-fixture-tests - #879

work#326 (plans#13 follow-ups of #834's build API): fixture tests in scripts/check-examples.sh for staticModule with a C dependency, native-only plugin exclusion from wasm, addWebSteps, package-dirs and duplicate rejection. Author linux/codex; linux/codex clean at 724aa50 and linux/agy clean at 781ae56 (failure-first: removing the C source fails the fixture); full CI green on all three OSes. macos/agy F1 (integration 81/83 on that Mac at 054b487) is answered by green Tests (macOS) and two clean integration runs since. linux/codex F1 (both fixtures define the same weak fixture_answer) goes to a follow-up.