NULL-aware after()

[0x054]

Parent

Part of #372 (ADR-0018).

What to build

after() understands nullable order keys and empty position slots. Crossing from a non-NULL leading key into the NULL bucket (the pinned-first conversation list) is one query. None is legal in every non-PK slot; the PK slot still cannot be empty. Column nullability is not consulted — a left_join'd NOT NULL related column may still be NULL when the relation is missing.

Lifts the nullable-order-key refusal from #393.

Acceptance criteria

• after((None, id)) continues through the NULL bucket of a leading nullable key (nulls="last" DESC — the Pinch shape).
• after on a non-NULL position includes the later NULL-bucket rows when null placement puts NULLs after the cursor.
• None in a non-PK slot is legal; None in the PK slot is a build-time error.
• left_join + None for a NOT NULL related column is a legal position (when traversal is in scope this is tested there; the expansion must already allow it).
• One expansion function renders the IS NULL tree from each order key's direction and null placement. Both dialects consume it. native resolves at render, not in Python.
• Backend-matrix result sets pin the bucket crossing. Rust unit tests pin the expansion combinatorics (direction × placement × after × NULL).

Blocked by

• after(position) on non-null root order keys #393

[0x054]

Landed on feat/position-paging via #399 (04723ec). GitHub will not auto-close from a non-default base.