Add separate compilation: extern fn, --emit-header, elb build coordinator

el-compiler/runtime/el_runtime.c

@@ -1853,6 +1853,29 @@ el_val_t fs_write_bytes(el_val_t pathv, el_val_t bytesv, el_val_t lengthv) {
     return 1;
 }
 
+// exec_command — run a shell command, return exit code (0 = success).
+// Used by elb and other El tooling to invoke subprocesses.
+el_val_t exec_command(el_val_t cmdv) {
+    const char* cmd = EL_CSTR(cmdv);
+    if (!cmd) return (el_val_t)(int64_t)-1;
+    int ret = system(cmd);
+    return (el_val_t)(int64_t)ret;
+}
+
+// exec_capture — run a shell command, capture stdout, return as String.
+// Returns "" on failure.
+el_val_t exec_capture(el_val_t cmdv) {
+    const char* cmd = EL_CSTR(cmdv);
+    if (!cmd) return el_wrap_str(el_strdup(""));
+    FILE* f = popen(cmd, "r");
+    if (!f) return el_wrap_str(el_strdup(""));
+    JsonBuf b; jb_init(&b);
+    char buf[4096];
+    while (fgets(buf, sizeof(buf), f)) jb_puts(&b, buf);
+    pclose(f);
+    return el_wrap_str(b.buf);
+}
+
 el_val_t fs_list(el_val_t pathv) {
     const char* path = EL_CSTR(pathv);
     el_val_t lst = el_list_empty();