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

README.md

@@ -198,7 +198,7 @@ func Main() -> int {
 | **CTFE** | `const func` — compile-time function execution |
 | **Trait Bounds** | `func Max<T: Comparable>(a: T, b: T) -> T` |
 | **Package Manager** | `bux add`, `bux install`, `bux.lock`, path + git deps |
-| **Tooling** | `bux new`, `bux build`, `bux run`, `bux check` |
+| **Tooling** | `bux new`, `bux build`, `bux run`, `bux test`, `bux check` |
 
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