DOCFIX-134: S8a -- FIP pool literals to lib-net.sh (single source of truth) ...

D-003 FIP pool (10.12.5.0-10.12.7.254) was hardcoded in phase-04-network-create
(env-overridable) and -verify (pinned). Both already source lib-net.sh; added
FIP_POOL_START/END there and referenced them. Gauntlet 39/39. S8b (KEYSTONE_VIP,
needs sourcing into client-critical scripts) + S8c (python cross-source) deferred
for operator decision -- bigger than the S estimate.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Phase 3 updates