feat(streams): allow selecting video probe stream

Add --video-stream and [encoding].video_stream so users can choose which
input video stream is probed for preset helper variables.

Use the selected stream for codec, pixel format, frame rate, output
pixel format, and GOP size metadata while leaving preset stream mapping
unchanged.

Validate CLI stream selectors strictly and ignore invalid config values
with a warning so bad configuration falls back to the default v:0
stream.

Document the new option in the README and man page, add verbose dry-run
output, and provide bash completion for common video stream selectors.

Closes #6.

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