Move Q-value stats, gradient norm, and training step from tracing::info!
(stdout-only, block-buffered, invisible mid-epoch) to Prometheus gauges
(atomic set, scrapable every 15s). Downgrade formatted step logs to
debug! level to eliminate allocation/serialization overhead in the hot path.
New gauge: foxhunt_training_step (current step within epoch)
Updated: foxhunt_training_q_value_mean/max, foxhunt_training_gradient_norm
now update every 500 steps instead of only at epoch boundaries.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
0355fb17bc