Use UDF file_extents() to read actual allocation descriptors instead
of assuming m2ts files are contiguous from file_start_lba. Dual-layer
UHD discs split large files across 70+ extents (~1 GB each) — the old
code created one extent from packet count which only covered the first
chunk, causing silent truncation at ~37%.
Also changed fill_extents() to return io::Result<bool> so read errors
propagate instead of being silently treated as EOF.
Matt Jackson
23990de768