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

library/runtime/runtime.c

@@ -1324,6 +1324,20 @@ const char* bux_socket_error(void) {
     return strerror(errno);
 }
 
+/* ============================================================================
+ * Test / Assert primitives
+ * ============================================================================ */
+
+void bux_exit(int code) {
+    exit(code);
+}
+
+void bux_assert(int cond, const char* file, int line, const char* expr) {
+    if (!cond) {
+        fprintf(stderr, "ASSERT FAILED: %s at %s:%d\n", expr, file, line);
+        exit(1);
+    }
+}
 
 /* ============================================================================
  * Cryptography primitives (OpenSSL)