_topology_compose_path returned a CWD-relative Path, so every
deploy/mutate/dry-run wrote decnet-topology-<id8>-compose.yml into the
process CWD (the install dir). Teardown computed the same relative path
against its own CWD, so when it differed the unlink() missed the orphan
and files accumulated forever.
Anchor to $DECNET_RUN_DIR (default /var/lib/decnet/topologies, tempdir
fallback) so write and teardown always agree regardless of CWD.