feat(streamio-ffmpeg): Add types for v3.0

[ksss]

Add RBS for the basic API of streamio-ffmpeg 3.0.

• FFMPEG module functions (logger, ffmpeg_binary, ffprobe_binary, max_http_redirect_attempts, which)
• FFMPEG::Error / FFMPEG::HTTPTooManyRequests
• FFMPEG::Movie (metadata readers, transcode, screenshot)
• FFMPEG::Transcoder
• FFMPEG::EncodingOptions (minimal, since it appears in argument types)

Types follow the implementation rather than the README where they differ, e.g. Movie#duration is Float | Integer (it is 0 when ffprobe reports an error) and Transcoder.timeout defaults to nil.

Tests are based on the examples in the README.

🤖 Generated with Claude Code

[github-actions]

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streamio-ffmpeg

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[ksss]

/merge