The video mode now paints a frame immediately: on `loadedmetadata` it
seeks to the initial scroll position instead of waiting for the first
scroll (the progress-0 loop guard used to leave some browsers black).
New `start` / `end` props (seconds, clamped to the real bounds) scrub
only a SEGMENT of a clip — progress 0->1 maps to `[start, end]`, `end`
defaults to the full duration. Time-based seek means frame rate is
irrelevant and duration stays browser-authoritative (read from
`loadedmetadata`, never passed by hand).
Demo: same-origin `/demos/video.mp4` sample (external URLs fail on
cross-origin range requests), live `start` / `end` / `smooth` controls,
API rows + snippet parity, README segment example.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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