Add getpid, exec_bg, spawn_thread, sleep_secs builtins; fix fs_write arity

bin/el/src/main.rs

@@ -2411,6 +2411,61 @@ fn dispatch_builtin(
             BuiltinResult::Handled
         }
 
+        // ── Process / thread builtins ─────────────────────────────────────────
+
+        "getpid" => {
+            stack.push(Value::Int(std::process::id() as i64));
+            BuiltinResult::Handled
+        }
+
+        "exec_bg" => {
+            let cmd_str = match stack.pop().unwrap_or(Value::Nil) {
+                Value::Str(s) => s,
+                _ => { stack.push(Value::Int(-1)); return BuiltinResult::Handled; }
+            };
+            let mut parts = cmd_str.split_whitespace();
+            let prog = match parts.next() {
+                Some(p) => p.to_string(),
+                None => { stack.push(Value::Int(-1)); return BuiltinResult::Handled; }
+            };
+            let args_vec: Vec<String> = parts.map(|s| s.to_string()).collect();
+            let pid = std::process::Command::new(&prog)
+                .args(&args_vec)
+                .spawn()
+                .map(|child| child.id() as i64)
+                .unwrap_or(-1);
+            stack.push(Value::Int(pid));
+            BuiltinResult::Handled
+        }
+
+        "spawn_thread" => {
+            let fn_name = match stack.pop().unwrap_or(Value::Nil) {
+                Value::Str(s) => s,
+                _ => { stack.push(Value::Nil); return BuiltinResult::Handled; }
+            };
+            let instr_arc = SERVE_INSTRUCTIONS.with(|si| si.borrow().clone());
+            let fn_arc = SERVE_FN_TABLE.with(|sf| sf.borrow().clone());
+            if let (Some(instr_arc), Some(fn_arc)) = (instr_arc, fn_arc) {
+                std::thread::spawn(move || {
+                    if let Some(&entry) = fn_arc.get(&fn_name) {
+                        run_sub_interpreter(&instr_arc, &fn_arc, entry);
+                    }
+                });
+            }
+            stack.push(Value::Nil);
+            BuiltinResult::Handled
+        }
+
+        "sleep_secs" => {
+            let n = match stack.pop().unwrap_or(Value::Nil) {
+                Value::Int(n) => n as u64,
+                _ => 1,
+            };
+            std::thread::sleep(std::time::Duration::from_secs(n));
+            stack.push(Value::Nil);
+            BuiltinResult::Handled
+        }
+
         // ── String utility builtins ───────────────────────────────────────────
 
         "str_replace" => {