feat: add selfhost-loop determinism check, golden tests, improve sema diagnostics
- 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
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