CARLA usability fixes (HUD, cleanup, substepping) + Ubuntu/2D defaults

[servinar]

Description

A bundle of small CARLA-related fixes plus one opinionated default change. Each commit is self-contained — happy to drop or split out individual ones if preferred.

Bug fixes:

• Allow Ubuntu's apport_python_hook in the excepthook check — On Ubuntu's system Python, apport_python_hook is installed as the default excepthook, which previously caused Scenic to warn and skip installing its own backtrace formatter. Treating it the same as exceptiongroup lets Scenic backtraces format correctly out of the box.
• Wire up HUD tick() and render() in the CARLA simulator — CarlaSimulation constructed a visuals.HUD but never called tick() or render(), leaving the panel permanently blank. Register on_world_tick for FPS counting and call hud.tick() and hud.render() each step alongside the camera render.
• Clean up residual CARLA actors before each simulation — When a previous simulation crashed or was interrupted, leftover walkers / vehicles / sensors could remain in the world and affect the next run. New _cleanupWorld() destroys leftovers at the start of createSimulation().
• Enable CARLA physics substepping — Sets substepping = True, max_substep_delta_time = 0.01, max_substeps = 10 (the configuration recommended by CARLA's docs for synchronous mode). Reduces tunnelling and improves collision behavior at higher speeds.

Opinionated default (open to discussion):

• Default --2d and mode2D to True. In our usage, the simulator integrations we work with require 2D compatibility mode anyway, so flipping the default removes the need to specify --2d on every invocation. Fully open to dropping this commit if the project would rather keep the existing 3D default.

Issue Link

N/A — issues encountered during development of CARLA-based driving scenarios.

Checklist

• I have tested the changes locally via pytest and/or other means
• I have added or updated relevant documentation
• I have autoformatted the code with black and isort
• I have added test cases (if applicable)

Additional Notes

The HUD wiring and the auto-cleanup of residual actors were verified end-to-end against a running CARLA server (Town01, ego rear-ended a stopped vehicle). The Ubuntu backtrace fix mirrors the existing exceptiongroup handling.