Kai confirmed the host is a dedicated LLM VPS where pi already runs as a child
of the systemd --user service; the systemd sandbox (ProtectSystem=strict,
ProtectHome=read-only, NoNewPrivileges, ...) is the isolation ceiling and no
extra sandbox complexity is wanted. bubblewrap is cancelled.
compose.yaml / Dockerfile were never runnable (no pi, no ffmpeg, env_file
pointing at a missing file) and are deleted; P2-4 is updated to reflect that.