feat: async-generator 语义（async function*），yield/await 共存 + next 返回 Promise

- emit 识别 generator+async 组合，模块同时标记 is_generator/is_async
- 新增 AsyncGeneratorState：帧快照 + 请求队列 + 当前请求
- next/return/throw 返回 Promise，请求入队逐条处理
- YIELD 让出经 promise unwrap 后交付（yield rejected promise 关闭生成器）
- AWAIT 挂起保留当前请求，微任务恢复后继续执行
- %AsyncGeneratorPrototype% / %AsyncGeneratorFunction.prototype% 内建
- session GC / promote 支持异步生成器状态盒
- 普通 async 函数嵌套调用隔离外层异步生成器上下文

crates/oxide_vm/src/session_gc.rs

@@ -98,6 +98,9 @@ impl SessionGc {
         if obj.is_async_obj() {
             edges.extend(crate::async_func::async_native_edges(obj));
         }
+        if obj.is_async_generator_obj() {
+            edges.extend(crate::async_generator::async_generator_native_edges(obj));
+        }
         // 遍历 upvalue cell 中的对象引用。
         for cell_ptr in obj.upvalues_slice() {
             if cell_ptr.is_null() {
@@ -174,6 +177,13 @@ impl SessionGc {
                 }
             }
         }
+        if obj.is_async_generator_obj() {
+            for value in crate::async_generator::async_generator_native_edges(obj) {
+                if value.is_string() {
+                    live.insert(value.as_string_ptr_mut());
+                }
+            }
+        }
     }
 
     pub(crate) fn mark(&mut self, vm: &Vm) {
@@ -286,6 +296,7 @@ impl SessionGc {
             freed_bytes += crate::generator::drop_generator_native(obj);
             freed_bytes += crate::promise::drop_promise_native(obj);
             freed_bytes += crate::async_func::drop_async_native(obj);
+            freed_bytes += crate::async_generator::drop_async_generator_native(obj);
 
             freed_bytes
         }
@@ -346,6 +357,8 @@ impl SessionGc {
                     crate::promise::clone_promise_native_with_rewrite(old_ref, new_ref, |value| value);
                 } else if old_ref.is_async_obj() {
                     crate::async_func::clone_async_native_with_rewrite(old_ref, new_ref, |value| value);
+                } else if old_ref.is_async_generator_obj() {
+                    crate::async_generator::clone_async_generator_native_with_rewrite(old_ref, new_ref, |value| value);
                 }
                 forwarding.insert(old_ptr, new_ptr);
                 freed_bytes += Self::drop_session_object_heap_data(old_ptr);
@@ -381,6 +394,10 @@ impl SessionGc {
                 crate::promise::rewrite_promise_native(obj, |value| rewrite_forwarded_value(value, &forwarding));
             } else if obj.is_async_obj() {
                 crate::async_func::rewrite_async_native(obj, |value| rewrite_forwarded_value(value, &forwarding));
+            } else if obj.is_async_generator_obj() {
+                crate::async_generator::rewrite_async_generator_native(obj, |value| {
+                    rewrite_forwarded_value(value, &forwarding)
+                });
             }
         }