Four fixes found from one question about two stray ffmpeg processes:

- an audio track is built at its length instead of padded up to it, so the
  mix cannot hang at 100% CPU (4/10) or silently come out a clip short (3/14)
- an ffmpeg we start can no longer outlive us (PR_SET_PDEATHSIG)
- an image load reserves its VRAM for its load window, so it is visible to the
  eviction ladder instead of handing out memory it has already taken
- the dtype default is chosen from a device that has been detected, so a text
  model stops loading fp32 on a CUDA card -- twice the VRAM, and no Flash
  Attention, which is why Qwen3.5-9B answered nothing since 2026-09-30

Plus `precision` is settable from the model page, where the quantization it
belongs with already was.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZnLhydX9QqUcfBbqLu