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/oaknode/src/nodes/generatorwithmerge.rs

@@ -90,7 +90,9 @@ impl GeneratorWithMerge {
 	/// The Rust model has no shader-job payload (see
 	/// [`crate::nodes::mathbase`]): the merged case pushes a null
 	/// texture handle marking a renderer-deferred `"mrg"` shader job,
-	/// and the un-merged case pushes `job` itself.
+	/// and the un-merged case pushes `job` itself. `job` is an opaque
+	/// oakrender texture handle (cross-module payload; null in the
+	/// deferred-job model) — see [`crate::value::NodeValue::Texture`].
 	pub fn push_mergable_job(
 		inputs: &crate::value::NodeValueRow,
 		job: crate::handle::CHandle,