DOCFIX-151: $DC selector convention for lib-net.sh/lib-hosts.sh (gap #1 closed) ...

Adds lib_net_select_dc() (dc0/dc1 no-op per D-101 inherited-layout ruling,
dc2 fails loud pending NetBox) and lib_hosts_select_dc() (dc0 no-op, dc1+dc2
both fail loud -- no per-DC host enrollment exists yet). Backward compatible;
21/21 new harness tests green; full gauntlet unaffected (A/B via git stash).
Unblocks the Stage 5 runbook's "high reuse" claim in the DC-DC deployment
workflow.

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