after(position) on non-null root order keys

[0x054]

Parent

Part of #372 (ADR-0018).

What to build

after(position) pages a query whose order keys are root columns, include the primary key, and are all non-nullable. position is the ordered tuple of those values; position_of(row) reads it; after(row) is sugar. The bound is exclusive. A nullable order key is a build-time error until #394.

Acceptance criteria

• after((ts, id)).limit(n) returns the next n matching rows after that position, declared order, exclusive of the position.
• position_of(row) returns the order-key tuple; after(row) equals after(position_of(row)).
• Order keys must include the model's primary key; PK-less models fail at build time. Do not append the PK silently.
• A nullable order key is a build-time error (lifted by the NULL-aware ticket).
• after + offset is a build-time error. count() drops the bound. Mutations reject it the same way they reject limit/offset.
• Wrong arity is a build-time error. A bound that matches no rows is an empty page.
• QueryIR fetch payloads carry one after bound with typed values. Golden vector pins the shape. count / mutate verb policy stays: bound dropped / paging keys omitted.

Blocked by

• Omitted null placement is last #392

[0x054]

Landed on feat/position-paging via #398 (fb2f5f1). GitHub will not auto-close from a non-default base.