Omitted null placement is last

[0x054]

Parent

Part of #372 (ADR-0017).

What to build

order_by without nulls= sorts NULLs last on every backend. nulls="native" is the explicit escape for a backend's own default (Postgres and SQLite are opposites). Every order key on the wire carries an explicit null placement — a missing key is invalid.

This is a breaking change for omitted nulls=: Postgres DESC and SQLite ASC flip (NULLs move to last).

Acceptance criteria

• nulls= accepts last, first, and native. Omitted means last.
• Omitted nulls= on a nullable column is NULLS LAST on both backends (cross-asserted result-set order).
• nulls="native" restores the documented dialect split and is not cross-asserted.
• Every order_by term on the QueryIR payload carries explicit nulls. QueryIR version bumps. Existing golden vectors are updated by hand.
• Invalid nulls= token is a build-time error.

Blocked by

None — can start immediately.