fix: selfhost CLI run command, lexer char literal, parser test path, calloc in runtime

- Fix lexer unterminated char error test (was treating 'a as lifetime)
- Fix parser test sample.bux path for root-dir execution
- Add bux_system() to runtime.c and run command to selfhost CLI
- Switch bux_alloc from malloc to calloc for zero-init
- Fix module flattening in Cli_Check/Cli_Compile to scan full decl list
- Make test passes fully, selfhost builds successfully

stdlib/runtime.c

@@ -22,7 +22,7 @@ char* bux_argv(int index) {
 
 /* Memory allocation */
 void* bux_alloc(size_t size) {
-    void* ptr = malloc(size);
+    void* ptr = calloc(1, size);
     if (ptr == NULL) {
         fprintf(stderr, "bux runtime: out of memory (alloc %zu bytes)\n", size);
         abort();
@@ -75,6 +75,11 @@ int bux_run_nim(const char* nim_file, const char* out_bin) {
     return system(cmd);
 }
 
+int bux_system(const char* cmd) {
+    if (cmd == NULL) return -1;
+    return system(cmd);
+}
+
 int bux_strlen_c(const char* s) {
     if (s == NULL) return 0;
     const char* p = s;