refactor: purge CHandle from module internals (M14 R5)

Module-internal object references are Rust types now (values, Arc,
Mutex); CHandle remains only at the oakengine C-ABI boundary:

- oakundo: the global stack holds UndoStack/UndoCommand values
  directly (stack token is the static's address)
- oaktimeline: marker/workarea boxes carry Arc<Mutex<T>>; commands
  share the same allocation through Arc clones (readers in oakengine
  stubs and the app's graphops updated to lock)
- oaktask/oakstorage: sessions, write-through bindings and the
  database backend pass ProjectArc; the Session drops its manual
  release bookkeeping; nodeutil keeps the CHandle<->Arc boundary
  conversion (release_project restored for the app)
- oakcodec: handle.rs deleted outright (no facade entry needed it);
  texture/block placeholders are unit structs
- oakrender: copier's project handle is an identity u64; alive-count
  machinery removed; handle.rs is make_owned/get/get_mut only
- oakplugin: the instance registry is gone (its unregister key never
  matched, leaking weak entries); handle.rs is the RefBox boundary type
- oaknode/oakcommon: only dead guard/borrow helpers removed; external
  payload handles (texture/processor) documented as the boundary

Flake hunts landed along the way: the audio recording test serializes
on the shared manager lock with a normalized state; the autocacher
cancel test uses a slow producer so cancellation is deterministic.

crates/oakplugin/src/instance.rs

@@ -86,8 +86,9 @@ pub struct RenderScale {
 	pub y: f64,
 }
 
-/// 插件实例。`Arc<RefBox<Instance>>` 管理生命周期；身份注册见
-/// [`crate::handle::Registry`]（param 桥按身份反查）。
+/// 插件实例。`Arc<RefBox<Instance>>` 管理生命周期（`RefBox` 为 facade
+/// 边界类型，见 [`crate::handle`]）；param 桥按 props 地址映射反查
+/// （[`crate::suites::param`]），节点绑定经 [`crate::node`] 身份注册表。
 ///
 /// `#[repr(C)]` + props 在偏移 0（句柄约定，见 [`crate::suites::tag`]；
 /// 实例期 effect/param-set handle 即 `&props`）。
@@ -123,8 +124,9 @@ pub struct Instance {
 	pub render_lock: std::sync::Mutex<()>,
 }
 
-/// 实例销毁路径：先通知 destroyInstance action，再摘除身份登记。
-/// （RefBox 归零时 Drop 触发——action 通知必须在对象析构前发出。）
+/// 实例销毁路径：先通知 destroyInstance action，再摘除 param 回写登记。
+/// （`Arc<RefBox<Instance>>` 归零时 Drop 触发——action 通知必须在
+/// 对象析构前发出。）
 impl Drop for Instance {
 	fn drop(&mut self) {
 		if !self
@@ -134,8 +136,6 @@ impl Drop for Instance {
 			self.notify_destroy();
 		}
 		crate::suites::param::unregister_params_of(&self.props as *const _ as usize);
-		crate::host::instance_registry()
-			.unregister(&self.props as *const crate::property::PropertySet as usize);
 	}
 }
 
@@ -954,8 +954,8 @@ impl Instance {
 		Ok(())
 	}
 
-	/// 销毁（destroyInstance action）。析构由 RefBox 驱动；
-	/// 此处只做 action 通知，幂等（[`Instance::drop`] 的
+	/// 销毁（destroyInstance action）。析构由 `Arc<RefBox<Instance>>`
+	/// 归零驱动；此处只做 action 通知，幂等（[`Instance::drop`] 的
 	/// `destroyed` 门保证只发一次）。
 	pub(crate) fn notify_destroy(&self) {
 		use crate::host::ACTION_DESTROY_INSTANCE;