fix: duration was exposed in microseconds but is expected to be in seconds

[benjaminrobinet]

Summary

• Video.duration is documented as seconds but TrackDecoder.duration was returning the raw microsecond value from the manifest, breaking seek(time) whose time is in seconds.
• The manifest keeps FSVTrack.duration in microseconds (matching WebCodecs conventions and chunk.timestamp); conversion to seconds now happens at the public duration getter boundary.
• The .seek() was expecting to receive a time in seconds

[vercel]

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