test(query): audit mutation and join capability gating

[scottescue]

Outcome

A query conformance audit identifies mutation, join, limit, returning, and dialect-specific clause paths that silently change meaning instead of returning build errors.

In scope

• Matrix PostgreSQL, MySQL, and SQLite behavior for update/delete joins and limits, returning, conflict clauses, and dialect-specific hints.
• Assert exact SQL on supported paths.
• Assert stable capability errors after concrete feature fixes land.
• Link uncovered behavior to one narrowly scoped implementation issue.

Out of scope

• Implementing UPDATE FROM, index hints, or conflict APIs in this audit issue.
• Duplicating existing concrete issues such as typed UPDATE FROM.

Acceptance criteria

• Every normative mutation/join capability has positive and negative dialect cases.
• No unsupported requested clause is silently removed or rewritten.
• Each uncovered implementation gap has one canonical owner.
• The matrix works for normal and prepared builds through the shared error contract.