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/oakrender/tests/common/mod.rs

@@ -45,17 +45,6 @@ impl Drop for ManagerGuard {
 	}
 }
 
-/// A non-null fake handle (ctx only — the ABI functions that accept
-/// borrowed handles only check `ctx` in this pass).
-pub fn fake_handle(seed: usize) -> oakrender::handle::CHandle {
-	oakrender::handle::CHandle {
-		ctx: seed as *mut std::ffi::c_void,
-		addref: None,
-		release: None,
-		abi_version: oakrender::handle::OAKRENDER_ABI_VERSION,
-	}
-}
-
 // ---------------------------------------------------------------------------
 // Host-symbol stand-ins (oakcore_* / fb_find_best_pix_fmt_of_list)
 // ---------------------------------------------------------------------------