EQL v3: make incorrect JSON functional indexes discoverable #980

For encrypted JSON containment, the indexable column expression is
eql_v3.to_ste_vec_query(column)::jsonb. Calling eql_v3.ste_vec(column) is
also legal and produces an indexable type, so PostgreSQL accepts a GIN index on
it even though containment queries can never use that index.

The repository now documents the correct per-domain index recipes, but the SQL
function that creates the trap still gives no local indication that it is an
element accessor rather than an index-term extractor.

Acceptance criteria

• COMMENT ON FUNCTION eql_v3.ste_vec(jsonb) identifies it as an element
  accessor, warns that it is not the containment index expression, and points
  to eql_v3.to_ste_vec_query(column)::jsonb.
• The generated SQL bundle preserves that comment.
• Index guidance and an EXPLAIN-backed regression test continue to use the exact
  expression that the containment operator inlines to.

Historical provenance: cipherstash/encrypt-query-language#417. That repository
is archived; this Stack issue is the canonical location for remaining work.