Position paging on traversed order keys and projected records

[0x054]

Parent

Part of #372 (ADR-0018).

What to build

after / before work when an order key is a traversed column or the query is a projected record. position_of on a model instance requires those relations populated; a decoded tuple does not. position_of on a projected record requires every order key to be in the projection. Grouped aggregates still fail the primary-key-in-order-keys rule.

Acceptance criteria

• order_by(lambda t: t.account.label).order_by(lambda t: t.id) pages with after / before from a tuple, without include().
• position_of on a model instance with a traversed order key requires that path populated; unpopulated is a build-time error.
• ProjectedQuery.after / before page records the same way instances page.
• position_of(Row) works when every order key is in the projection; missing an order key is a build-time error (pass a tuple).
• after / before on a grouped aggregate is a build-time error (PK is not an order key of a GROUP BY). Aggregate / expression order keys remain a build-time error.
• Backend-matrix covers a traversed happy path and a projected-record happy path. Build-time suite covers the unpopulated / missing-key / aggregate errors.

Blocked by

• before(position) #395

[0x054]

Landed on feat/position-paging via #403 (562ea92). GitHub will not auto-close from a non-default base.