Problem: the Matroska demuxer mapped A_MPEG/L2 and A_MPEG/L3 only, so a
Layer I track was reported as an unsupported codec even though the MP1/MP2
decoder (#809) reads Layer I.
Fix: add Codec::Mp1 for A_MPEG/L1 and route it through the same packet
duration and decoder path as Mp2. Test remuxes synthetic Layer I frames
into Matroska with ffmpeg and compares our decode (max error 3e-5).
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
vangel76