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)

library/std/Test.bux

@@ -0,0 +1,44 @@
+module Std::Test {
+
+extern func bux_exit(code: int);
+extern func bux_assert(cond: int, file: String, line: int, expr: String);
+
+func Test_Exit(code: int) {
+    bux_exit(code);
+}
+
+func Test_Assert(cond: bool) {
+    bux_assert(cond as int, "", 0, "");
+}
+
+func Test_AssertEqInt(a: int, b: int) {
+    if a != b {
+        PrintLine("ASSERT_EQ FAILED:");
+        PrintInt(a);
+        PrintLine(" != ");
+        PrintInt(b);
+        bux_exit(1);
+    }
+}
+
+func Test_AssertTrue(cond: bool) {
+    if !cond {
+        PrintLine("ASSERT_TRUE FAILED");
+        bux_exit(1);
+    }
+}
+
+func Test_AssertFalse(cond: bool) {
+    if cond {
+        PrintLine("ASSERT_FALSE FAILED");
+        bux_exit(1);
+    }
+}
+
+func Test_Fail(msg: String) {
+    PrintLine("FAIL:");
+    PrintLine(msg);
+    bux_exit(1);
+}
+
+}