Address @vanceingalls review on #851:
1. validateMetadata's codec/format check read 'rc.codec !== undefined &&
rc.format !== undefined && rc.format !== "mp4"'. The behavior was
correct (omitted format defaults to mp4 downstream so codec is legal)
but relied on the reader knowing that default. Normalize 'effectiveFormat
= rc.format ?? "mp4"' before the comparison so the intent reads
directly.
2. The mp4-h264-sdr sibling carries inline rationale for the no-audio
choice (AAC frame quantization extends container.duration past
nb_frames/fps and trips the harness PSNR sampler) and the chunk-seam
mapping (crossfade window 0.9-1.1s straddles frame 30). mp4-h265-sdr
stripped both. Carry them back so the two fixtures stay parallel.
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James