Add lightweight browser video previews with explicit clock mappings
Add 10 browser-playable H264 MP4 previews for both episodes: left/right front and rear lenses plus head. Previews fit 768x768, are capped at 30 fps (head retains its original approximately 29.97 fps), use CRF 27 and faststart, and total approximately 434 MB. Fisheye lens views remain unstitched. Original recordings, all pose rows, track processing and SPZ assets/transforms are unchanged. Add only episodes[].videos to metadata, per-video provenance receipts and updated SHA256SUMS/README. Eight hand lens videos keep the existing native-video-to-episode clock offsets. Head videos explicitly use timing: null and independent playback; no missing offset is inferred. Source revision is 79db41b76b93d240045020a77e3e1e4976bb61e5. Presentation previews may drop frames and are not intended as verification exports.