Corrects the changelog claim that the old guard messages only named env keys:
they named fields too, it was the remediation advice that pointed at env keys.
Records the human approval of the design, the mutation-testing evidence from
the independent verifier, and the same-class gap it found: 14 checks (redis_url
presence, telemetry paths, enum validity) still live only in from_env, while
client.py asserts they were already validated. Deliberately out of scope here.
Patch release for the settings invariant fix. The changelog carries a separate
'behaviour tightening' section because a patch number gives downstreams no
warning that construction can now raise where it previously did not.
iomgaa