Calls lavf_options() post mpv.MPV() init and writes each entry into
the demuxer-lavf-o property. This is the consumer side of the split
helpers introduced in the previous commit. Verified end-to-end by
launching the GUI: mpv constructs cleanly and m['demuxer-lavf-o']
reads back as {'protocol_whitelist': 'file,http,https,tls,tcp'}.
Audit-Ref: SECURITY_AUDIT.md finding #2
Severity: High