A stale client can only echo the stored model_version, never raise it. The
'unchanged wall catalogue' refusal now applies only when the submitted model
is not above the stored one; the first v9 write over a v8 document with an
orphan open_span/open_to legitimately drops the legacy projection without
touching the catalogue and passes. The regression pair fixture is produced
by the real writers (stored: v1.68.0-beta.2, sent: current migration) and is
pinned on the frontend byte-for-byte so it cannot drift.
Issue: #319
User-Visible: yes