Sibling case to #308: when a fragmented MP4 segment begins with a sidx
box (no ftyp/styp/moof at the file root), Mp4InputFormat._canReadInput
returned false, causing Input.getTracks() to throw
UnsupportedInputFormatError.
This shape is standard in CMAF when the DASH on-demand profile is used
or when HLS playlists are derived from one (Vimeo's vod-adaptive-ak CDN,
for example). The segment is still a valid ISOBMFF fragment — the sidx
just sits before the moof.
Adds 'sidx' alongside the existing 'moof' check so the format probe
accepts these segments, and a focused regression test that fails on
the previous behaviour.
