Changes

fix: skip topics with unavailable message packages

Allow instant and retro recording to continue for usable topics when a
discovered topic's ROS message package is unavailable. Reject recording
only when no usable subscription remains, and expose per-topic warnings.

Bound and collapse long web diagnostics, group failures by missing package,
and avoid repeating unchanged subscription errors in the worker log.

Add subscription-readiness regression tests and document how message types
are discovered and how to diagnose missing packages.