Identifier downcase change for LATIN1, in v19

Opus 5 found that, under LATIN1, psql running "\d TABLENAME" can find a
different table than the one "SELECT * FROM TABLENAME" reads. Commit 87b2968
"downcase_identifier(): use method table from locale provider" changed this.
While the LLM likes the new backend behavior and argues for making the
frontend match it, I disagree. Identifier downcasing behavior is, in a loose
sense, part of the frontend-backend protocol. I doubt more locale-aware
identifier downcasing could be worth the compatibility break. The backend
should return to v18 identifier downcase behavior.

If new downcase behavior were worthwhile, the change would need to be clearly
release-noted, so other clients with psql-like logic can cope. It would also
entail making psql understand both generations of downcasing behavior, so psql
can do the right downcasing for older server versions it supports.

I'm attaching the LLM's test+report.