Researcher experience: friction points using comma2k19 / old rlogs

### Describe the bug

(Posting under "PC bug report" as it's the closest template — this is researcher/dataset
UX feedback rather than a bug.)

Context: we're academic researchers studying driver monitoring and takeover behavior.

1. Old logs: deprecated fields & migration discoverability
In comma2k19-era logs, the fields are hidden: `controlsState`'s
enabled/active/state live under the `deprecated` group, and driver monitoring is
`driverStateDEPRECATED`.
2. Dataset cards: document which streams are populated
In comma2k19 the driver monitoring stream exists but is all zeros (DM model wasn't
running on those devices) — this isn't mentioned anywhere, so researchers interested
in driver state only find out after decoding. A short "dataset card" per release
(which messages are populated, at what rates, known gaps) would set expectations and
make the datasets easier to cite in academic work.
3. Suggestion: a minimal research quickstart
One documented example — rlog → pandas DataFrame with common channels (enabled,
vEgo, steeringPressed, driver state probs, onroad events) — would open the datasets
to a much wider research audience. Happy to contribute our decoding snippet as a
starting point.

### OS Version

Mac

### openpilot version or commit

_No response_

### Additional info

_No response_

The issue identifies comma2k19-era rlogs, deprecated fields, and a possible research quickstart, but names no files or tests. Start by reading the dataset and rlog documentation and checking which streams are populated in comma2k19. Done means documenting the legacy field locations, dataset stream coverage and gaps, and a reproducible rlog-to-pandas example; the issue does not specify the expected documentation location.

Written by the indexing model from the issue text.