The BRouter host's trails user is non-root without sudo, so osmium-tool can't
be apt-installed. Ship osmium.Dockerfile (built on first use) and run osmium in
a container with the work dir bind-mounted (native I/O, negligible overhead on
Linux). python3/curl/gzip/sha256sum remain on the host. Correct the README
disk figure to the /home free space (~595 GB).
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
