chore: 消除所有编译器警告

- 移除unused方法: frame(), get_constant(), emit_i16_placeholder
- 移除LoopContext未使用字段: start_ip, scope_depth
- Vm字段改为私有: frames, open_upvalues
- is_const字段添加#[allow(dead_code)]
- 测试: 327/327 通过 | 0 warnings

aster-core/src/vm/vm.rs

@@ -46,7 +46,7 @@ pub struct Vm {
     pub stack: Vec<Value>,
 
     /// Call frames
-    pub frames: Vec<CallFrame>,
+    frames: Vec<CallFrame>,
 
     /// Script-level globals (top-level let bindings)
     pub globals: Rc<RefCell<HashMap<String, Value>>>,
@@ -61,7 +61,7 @@ pub struct Vm {
     pub current_dir: String,
 
     /// Open upvalues (tracked so closures share the same upvalue object)
-    pub open_upvalues: Vec<Rc<RefCell<UpvalueObj>>>,
+    open_upvalues: Vec<Rc<RefCell<UpvalueObj>>>,
 
     /// VM-compiled closures: maps Function Rc pointer → Closure data
     closures: RefCell<HashMap<*const crate::interpreter::Function, Rc<Closure>>>,
@@ -602,10 +602,6 @@ impl Vm {
     // IP management
     // ========================================================================
 
-    fn frame(&self) -> &CallFrame {
-        self.frames.last().unwrap()
-    }
-
     fn frame_mut(&mut self) -> &mut CallFrame {
         self.frames.last_mut().unwrap()
     }
@@ -917,13 +913,6 @@ impl Vm {
         }
     }
 
-    fn get_constant(&self, idx: usize) -> Result<Value, RuntimeError> {
-        let frame = self.frames.last().unwrap();
-        frame.closure.proto.constants.get(idx).cloned().ok_or_else(|| RuntimeError::RuntimeError {
-            message: format!("Constant index {} out of bounds", idx),
-            token: None,
-        })
-    }
 
     // ========================================================================
     // Upvalues