Fix Snap-to-EE segfault: dispatch to physics thread (#17)

The _on_snap callback ran on viser's websocket thread and called
arm.get_ee_pose() which accesses MuJoCo data (mj_forward +
site_xpos). Concurrent access from the wrong thread causes a
segfault.

Fix: dispatch the MuJoCo read through event_loop.run_on_physics_thread,
same pattern as _activate_teleop. Viser GUI updates (gizmo position)
are thread-safe and stay on the websocket thread.

Fixes ada_mj#17

Author: siddhss5

src/mj_viser/teleop_panel.py

@@ -298,7 +298,13 @@ def _on_activate(event) -> None:
 
         @self._snap_btn.on_click
         def _on_snap(event) -> None:
-            ee_pose = self._arm.get_ee_pose()
+            def _do_snap():
+                return self._arm.get_ee_pose()
+
+            if self._event_loop is not None:
+                ee_pose = self._event_loop.run_on_physics_thread(_do_snap)
+            else:
+                ee_pose = _do_snap()
             self._gizmo.wxyz = xmat_to_wxyz(ee_pose[:3, :3].flatten())
             self._gizmo.position = tuple(ee_pose[:3, 3])