fix(client): version-skew warning + compiled-XML model fallback

[jonathanembleyriches]

Problem

A client built with `local_model=False`, or one whose MJB fails to load (the MJB binary format is version-locked to the exact MuJoCo release that saved it), ends up with empty `art.joints` / `art.actuators` and no warning. Every name-based consumer then no-ops: the RoboJuDo policy adapter logs `dof joint 'X' not found in art.joints` for all 29 DoFs, drops all ctrl, and the robot stands still while the pipeline steps. The dashboard shipped exactly this state via a temporary `local_model=False` bypass, which is what URLab-Sim/UnrealRoboticsLab#76 ran into.

Changes

• MuJoCo version skew warns (major.minor compare) instead of raising `URLabVersionMismatch`. The class stays exported but is no longer raised.
• MJB load failure falls back to building the local model from the handshake's `mjcf_compiled` + `vfs_assets` (XML parses across MuJoCo versions). If the reply carried no XML, the client refetches the handshake once with `include_assets=true`.
• A handshake that yields articulations but no model now logs a loud warning that joint/actuator maps are empty.
• Removed the `mujoco_version_check=False, local_model=False` bypasses from the dashboard connect and the integration conftest.

Testing

uv run pytest tests --ignore=tests/integration
188 passed in 62.37s

4 new tests: skew warns not raises, bypass stays silent, unloadable MJB falls back to compiled XML, unloadable MJB without XML warns loudly. Verified live end to end: dashboard -> BeyondMimic on a G1 scene resolves all 29 joints and the robot tracks the policy.