R3 RULED: finish the jumbo underlay; first recorded MTU budget verdict ...

GA-R5: question and exact utterance quoted, dated, pushed before dependent work.
Operator utterance: "Raise the two lagging segments to 9000 (Recommended)".

Recorded as a D-101 RULING NOTE. D-102 is the original MTU sub-policy but is
MERGED INTO D-101 and its body directs amendments there.

MEASURED BEFORE ASKING, which materially re-framed the question. D-101 requires
"the measured underlay MTU is a Phase-0 gate -- do not assume jumbo", and
scripts/dc-dc-mtu-geneve-budget.sh had existed without ever being run to a
recorded verdict. Running it both ways, plus measuring the host bridges, showed
the jumbo branch is nearly complete already rather than a large project:
- Every vcloud MESH leg is ALREADY 9000, including mesh-vr1-dc0-vr1-dc1
  (virbr5), the inter-DC path itself. All six plane bridges on both racks: 9000.
- The four 1500 legs are the D-125 SIMULATED-ISP uplinks. They model the
  internet, D-125's egress gate is defined against them, and they must STAY
  1500. Not a defect.
- Exactly TWO segments lag: enp1s0 inside both containment VMs, and all 17 MAAS
  VLAN records.

The MAAS record is the one that bites silently: MAAS renders VLAN MTU into node
netplan, so a jumbo bridge beneath a 1500 record still yields 1500 node
interfaces. Jumbo bridges alone do not deliver a jumbo underlay.

Why (a): tenant MTU stays 1500, so no per-charm MTU coordination is needed at
all. Option (b) required ovn geneve + tenant-network MTU + amphora to agree
exactly and permanently across both DCs, and NOTHING in this repo checks MTU --
neither cloud-assert.sh nor provider-bundle-check.py has any MTU assertion -- so
drift would be silent, which is precisely what D-101 calls "the classic
nested-OpenStack failure mode". Option (c) was the worst: jumbo at the plane
layer with a still-capped transit throttles D-108 rbd-mirror and radosgw
multisite behind a chokepoint invisible where an operator would look.

COUPLING TO R2, recorded because it changes the arithmetic: the 56-byte overhead
is the IPv6 figure and applies BECAUSE dual-stack was ruled. Under v4-only it
would have been 42, and the 1500-underlay tenant MTU 1458. R2 and R3 are not
independent.

Execution is a SEPARATE gated step and is NOT authorised by this ruling. The
verification owed is BEHAVIOURAL: an end-to-end large-frame test with DF set
across the inter-DC path. An interface claiming 9000 is not proof a 9000-byte
frame survives the path -- the assert-on-content rule applied to MTU.

Also worth recording: invoked bare, the budget script correctly REFUSES ("FAIL:
--underlay-mtu is REQUIRED -- no default, measure it this session"). It does not
guess. That discipline is why this verdict is trustworthy.

Revert: git revert this commit; the ruling note and capture are additive.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HvCyrwvYTTcDYnRErfMsNf