refactor: 用 Value::get/set 简化属性访问，消除直接 borrow/borrow_mut 调用

- Value::set 签名改为接受 &str 并返回 RuntimeError，与 evaluator 对齐
- eval_get/eval_set/eval_index_get/eval_index_set 中统一使用 get/set
- 去掉 eval_index_get 和 eval_index_set 中不再使用的 map 绑定

Co-Authored-By: Claude <noreply@anthropic.com>

src/interpreter/mod.rs

@@ -33,6 +33,28 @@ pub enum Signal {
     Continue,       // continue 信号
 }
 
+impl Value {
+    pub fn get(&self, key: &str) -> Option<Value> {
+        match self {
+            Value::Object(obj) => obj.borrow().get(key).cloned(),
+            _ => None,
+        }
+    }
+
+    pub fn set(&self, key: &str, val: Value) -> Result<(), crate::error::RuntimeError> {
+        match self {
+            Value::Object(obj) => {
+                obj.borrow_mut().insert(key.to_string(), val);
+                Ok(())
+            },
+            _ => Err(crate::error::RuntimeError::RuntimeError {
+                message: "Only objects have properties".to_string(),
+                token: None,
+            }),
+        }
+    }
+}
+
 impl std::fmt::Debug for Value {
     fn fmt(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
         match self {