feat: bux test command + Std::Test module

- Add 'bux test' CLI command: discovers .bux files in tests/, compiles
  each into a temp package, runs them, reports pass/fail
- Add Std::Test module with Assert, AssertEqInt, AssertTrue,
  AssertFalse, Fail
- Add bux_exit and bux_assert C runtime primitives
- Update README.md and docs/BuildAndTest.md

docs/BuildAndTest.md

@@ -89,6 +89,9 @@ Output = "Bin"
 ./buxc run
 ./buxc run ./myproject
 
+# Run tests
+./buxc test
+
 # Clean build artifacts
 ./buxc clean
 ```