DOCFIX-166: $DC parameterization for reenroll-hosts/carve-host-interfaces/phase-00-maas-standup (gap #15, CLI half) ...

Each script gained an opt-in $DC env var, calling lib_net_select_dc/
lib_hosts_select_dc immediately after sourcing (unset $DC is byte-for-byte
unchanged VR0/DC0 behavior -- verified directly, no fixtures needed since
the selector calls sit before any MAAS call). DC=dc1 bash
scripts/phase-00-maas-standup.sh is now a real, correct, gated call
(D-101 makes dc1's target identical to dc0's); DC=dc1/dc2 for the other
two scripts still fails loud at the host selector until real per-DC host
data exists. New/extended test cases across three harnesses. Six of gap
#15's seven sub-findings remain open -- said plainly, not overclaimed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QrcJx8TUar7pYAvpGJw57A