Adaptive curriculum

[rafapi]

• Stage 0 warms up on base+medium only (hard stays off).
• Stage 1 unlocks once the first success stage is met, introducing a small proportion of the hard split.
• Stage 2 raises the hard share while reducing medium.
• Stage 3 requires the hard split’s own success metric before bumping hard exposure further.
• Stage 4 (tail phase), bringing the hard proportion above half once the hard metric clears its final gate.

All thresholds and patience values live in the config; stages re-check every stats publish and only advance after the criteria hold for the configured patience window.