chore: retain maintenance Flow merge in B0

[xor-machine]

Summary

Synchronize the B0 maintenance Group with its newly merged Flow commit before integrating the independent Docker-fixture correction.

Motivation / Context

Flow now contains the accepted maintenance Group through merge commit 8b4c65a5fece400476a2aea415e250c6eac4d213. The Group must retain that protected-parent history before its remaining fixture slice can be accepted.

Changes

• Add one deterministic two-parent synchronization commit.
• Preserve an identical tree while making the current Flow merge commit an ancestor of the Group line.

Testing

• git diff --exit-code 701b67b8c4690411ff91c4961c4f05d4bc6bad99 e71ce2cf324fe52b18582217b56383b9f27e01a5 passed with an identical tree.
• git merge-base --is-ancestor 8b4c65a5fece400476a2aea415e250c6eac4d213 e71ce2cf324fe52b18582217b56383b9f27e01a5 passed.
• git diff --check 701b67b8c4690411ff91c4961c4f05d4bc6bad99 e71ce2cf324fe52b18582217b56383b9f27e01a5 passed.

Evidence

Cycle 1

• Route: slice/group/b0-ci-control-plane/sync-flow-8b4c65a5fece -> group/b0-ci-control-plane
• Base: 701b67b8c4690411ff91c4961c4f05d4bc6bad99
• Head: e71ce2cf324fe52b18582217b56383b9f27e01a5
• Scenarios: retained-history Group synchronization after maintenance Flow integration
• Assertions: exact two parents; current Flow ancestry; identical pre- and post-sync trees; no protected-branch write
• RED predecessor: 701b67b8c4690411ff91c4961c4f05d4bc6bad99
• RED: current Flow merge commit was not an ancestor of the pre-sync Group head, rc=1 classification=assertion-failure
• GREEN: exact parent, ancestry, tree-identity, and whitespace custody checks passed, rc=0 classification=success
• Product mutation: detected omission or swapping of either synchronization parent through the exact-parent custody oracle, rc=1 classification=assertion-failure
• CI mutation: detected attempted reuse of the prior Group or Flow head and requires a complete exact-head campaign, rc=1 classification=assertion-failure
• Runner: isolated clean Group checkout; Codex xhigh deterministic custody coordinator; no worker or Serena required for the identical-tree synchronization
• Duration: deterministic synchronization and custody checks completed in under one minute
• Cleanup: no product fixture or mutation state was created; the checkout remains clean
• Artifacts: exact parents 701b67b and 8b4c65a; synchronization head e71ce2c
• Unverified: complete hosted exact-head CI and CodeQL are pending for synchronization head e71ce2c