Fade the rendered mp3 out instead of stopping abruptly

renderState now applies an ffmpeg afade over the last min(4s, 10% of length) so CLI renders end with a fade to silence rather than a hard stop. (The video graft already fades; this brings the standalone audio in line.)

▶ Play it: Neon Fiscal Year → Benthic Patience → Silken Reentry — a constellation across three families — the path is the mix https://stellate.app/?genre=citypop&seed=2671&path=1530.6221,586.10667,1376.19565&xdur=0.056

75acaca https://github.com/aboard-io/stellate/commit/75acacafb6169696ffc52aea2b173315f5a36326