Document the three undocumented releases (2.7.5 embed guidance + online
value function + mid_scene_guide; 2.7.6 value-function ascent + rating
overhaul; 2.7.7 reactive rescue + Batch Training) and bring the Scene
Chain Sampler, Hybrid Euler-2S, and Studio docs in line with current
parameters and features.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>