DOCFIX-162: MTU/geneve + Ceph disk-budget calculator scripts (gap #7) ...

scripts/dc-dc-mtu-geneve-budget.sh reproduces D-101's own worked example
exactly (1500-56=1444) and adds an IPv6-minimum-link-MTU sanity floor.
scripts/dc-dc-ceph-disk-budget.sh computes size=3-vs-size=2 fit against a
measured total disk, requiring --backup-overhead-fraction explicitly (no
invented default) and naming size=2 as an operator-logged decision, never
silently applied. 19/19 and 16/16 tests.

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