3D viewer: colour cameras by in_view coverage + shade overlap

[bamdadd]

The 3D viewer draws every camera the same colour. Make coverage legible: colour or highlight each camera by whether it currently sees the object, and shade the overlap regions.

Scope

• For a chosen frame (or across the animation from the sibling animate issue), colour each camera frustum by its in_view flag for that frame (e.g. green = sees the object, grey = blind), read straight from the manifest's per-camera in_view.
• Optionally draw a light ray/line from each seeing-camera to the object, so overlap (2+ cameras on one object) is obvious.
• Add a small legend / per-frame coverage count.

Acceptance

• On the handoff_ltr scene, a rendered frame clearly shows the left camera green early and grey late, the right cameras the reverse, and the wide camera green throughout.
• Reads in_view from the manifest (no reprojection in the viewer). ruff + mypy clean.

Pointers

• scripts/view_scene_3d.py, src/multicam_sim/handoff_ltr.py (its test documents the exact coverage pattern), manifest entities[*].frames[*].points[*].per_cam[*].in_view.