Improve async example and remove Network demo code

- Move SimToView and ViewToSim dataclasses from async.py to viewer_protocol.py (adding a user_data field) to consolidate IPC protocol definitions.
  - Remove the simulated Network class and latency UI controls from async.py to simplify the example and focus on core multiprocessing communication.
  - Use parser.parse() in main() for cleaner and more direct MuJoCo model loading.
  - Add cancel_join_thread() to multiprocessing queues and a join timeout to prevent atexit handlers from blocking during process shutdown.

PiperOrigin-RevId: 932327625
Change-Id: If91777dba934814b3f9faa17f9055869920575d7

python/mujoco/experimental/studio/viewer_protocol.py

@@ -16,10 +16,12 @@
 StudioApp uses the protocol for convenience methods that accept any viewer.
 """
 
+import dataclasses
+from typing import Any
 from typing import Protocol
-
 import mujoco
 from mujoco.experimental.studio import ux
+import numpy as np
 
 GFX_MODES = (
     'classic',
@@ -33,6 +35,27 @@ GFX_MODES = (
 )
 
 
+@dataclasses.dataclass
+class SimToView:
+  """A message sent from the simulation to the viewer."""
+
+  model: mujoco.MjModel | None = None
+  state: np.ndarray | None = None
+  state_sig: int = 0
+  user_data: dict[str, Any] = dataclasses.field(default_factory=dict)
+
+
+@dataclasses.dataclass
+class ViewToSim:
+  """A message sent from the viewer to the simulation."""
+
+  state: np.ndarray | None = None
+  state_sig: int = 0
+  reset: bool = False
+  send_rate: float = 60.0
+  user_data: dict[str, Any] = dataclasses.field(default_factory=dict)
+
+
 class Viewer(Protocol):
   """Structural interface for any viewer."""