- Makefile: new targets `test-golden` and `selfhost-loop`
- selfhost-loop verifies C codegen determinism (2-pass bootstrap)
- test-golden compares C output against expected.c golden files
- Updated clean-all to clean new build dirs
- tests/golden/hello/: first golden test (hello world)
- src/sema.bux:
- Fix: import stubs only registered if symbol doesn't already exist
- Improve: error message now includes identifier name
