Simplify trajectory capture scopes

[bradhilton]

Summary

• add art.no_capture() as a ContextVar capture barrier that hides enclosing trajectories while allowing explicit nested capture
• remove the unused TrajectoryGroup context manager, current_trajectory_group(), and implicit trajectory-to-group collection
• keep art.trajectory_group() as a pure concurrent aggregator and isolate iterable materialization, validation, and child execution from ambient capture

Validation

• capture suite: 70 passed
• complete trajectory suite: 268 passed
• full ty check src tests passed
• Ruff, formatting, lock consistency, and sdist/wheel build passed

The removed group-context API was introduced after the latest published release and has no repository consumers.