Process route groups in parallel using background subshells.
Each group runs independently with its own pref base computed
from the group's alphabetical index. Table allocation in
/etc/iproute2/rt_tables is protected by flock to prevent races.

On local-test with 3 groups: 163% CPU utilization confirms
parallel execution.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>