HiFi assembles FLAC from HLS segments and demuxes with `ffmpeg -c copy`,
which preserves total_samples=0 in the STREAMINFO of Tidal's fragmented/
streamed FLAC. Every audio frame is present and the file plays fine, but
mutagen computes length = total_samples / sample_rate = 0, so the integrity
check rejected it as 'zero-length audio' and quarantined nearly every HiFi
download. Users confirmed the quarantined files play normally once restored.
Length 0 is not proof of corruption at that point: the file already passed
the size gate, was identified as a real audio format, and has a valid info
block — a genuinely empty/truncated/stub file fails one of those earlier
checks instead. Treat length 0 as 'length unknown': accept the file and skip
the duration cross-check we can't perform without a length. mutagen never
decoded/validated frame data anyway, so this doesn't weaken real corruption
detection — size, parse, format, info-block, and duration-drift guards all
remain.
Tests: a large valid-parse length-0 file (streamed-FLAC signature) is now
accepted; a tiny length-0 stub still fails (size gate fires first).
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