The latent preparation stage was deriving num_frames from
batch.num_seconds * fps + 1 unconditionally and ignoring
batch.num_frames. num_seconds was never set anywhere (no plumbing in
SamplingParam or ForwardBatch), so every generation defaulted to 4
seconds = 101 frames, regardless of what the caller passed via
SamplingParam.num_frames.
Now we prefer batch.num_frames when it's a sensible video length (>1)
and fall back to the seconds-based derivation when the caller explicitly
passes num_seconds. Backward-compatible with the production preset
(num_frames=101 == 4*25+1). Fixes the SSIM test budget — it was running
at 101 frames despite asking for 26.
All 8 runnable magi-human parity tests still pass.
will
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