Collision detection: CARLA collision sensor + Newtonian 2D collision physics

[servinar]

Description

Adds collision detection in two places — a CARLA collision sensor wrapped as a Scenic-usable CollisionSensor, and pairwise 2D collision physics in the Newtonian simulator.

CARLA:

• New CarlaCollisionSensor wrapping sensor.other.collision, exported from the world model as CollisionSensor. Tracks collision_history, has_collision, and the most recent event's intensity / impulse, with reset() for between-simulation cleanup.
• CarlaSimulation.step() learns about event-based sensors: rather than blocking on sensor.frame != current_frame (which never matches for sensors that only fire on events), bump the frame number via updateFrame() and skip the wait. Sensors with a reset() method are reset on simulation teardown.
• Auto-attach a collision sensor to the ego in createObjectInSimulator so users can write terminate when ego.sensors['collision'].has_collision without boilerplate.
• Add a colored "Collision:" status line to the CARLA HUD via a new (label, value, color) tuple format; green for none, red for hit + intensity.

Newtonian:

• 2D pairwise collision detection via shapely polygon intersection on _corners2D, resolved with an SAT-derived collision normal. Apply impulse along the normal scaled by mass and restitution, and split penetration depth between the two objects.
• Optional enable_crash_physics (default on) marks colliding cars and pedestrians as crashed and applies post-impact dynamics: heavy friction, momentum transfer for car-pedestrian hits, randomized spin for car-car crashes. Per-object mass defaults to 1500 kg for cars and 70 kg for pedestrians.

Issue Link

N/A

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

• Verified the CARLA path end-to-end against a running CARLA server (Town01): ran a scenario without with sensors {...}, drove the ego forward via SetThrottleAction(1.0), and confirmed terminate when ego.sensors['collision'].has_collision fires when the ego rear-ends a stopped vehicle. Collision history populates with frame, other actor type, and impulse intensity.
• The HUD "Collision:" line is composed by this PR but is only blitted once step() actually calls hud.tick() and hud.render() — that wiring lives in companion PR CARLA usability fixes (HUD, cleanup, substepping) + Ubuntu/2D defaults #462. Until both land, the line is built but not rendered.
• These are two simulator changes in one PR for thematic reasons; happy to split if preferred.