Add Trackio trace logging

[abidlabs]

Hi folks! This PR adds trace logging via Trackio, the free, local-first experiment tracking library from Hugging Face 🤗

This PR follows ART's existing model.log(...) pattern for trajectory and metrics logging, specifically I did this:

• added report_metrics=["trackio"] support for Trackio metric logging
• added logging trajectories as trackio.Trace records with split/step/group/sample metadata
• added a helpful import error if Trackio logging is enabled but trackio is not installed
• documented Trackio trace logging and added tests with mocked Trackio

Here's what it looks like:

Duplicate-work check

I checked open PRs with these searches before opening this PR:

• trackio Trace
• Trackio trajectory
• trajectory traces

No open PRs matched this work.

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