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

tests/golden/hello/src/Main.bux

@@ -0,0 +1,5 @@
+import Std::Io::PrintLine;
+func Main() -> int {
+    PrintLine("Hello, Bux!");
+    return 0;
+}