feat(query): after(position) for non-null root order keys

[0x054]

Summary

• after(position) pages a query whose order keys are root columns, include the primary key, and are all non-nullable. The bound is exclusive. position_of(row) reads that tuple; after(row) is sugar for after(position_of(row)).
• QueryIR v13: fetch payloads carry one optional after bound as typed kind/value nodes (omitted when unset). count() drops paging the same way it drops limit/offset; update/delete reject after. after + offset is a build-time error (two starts).
• Rust expands one exclusive stepwise compare — (a > :a) OR (a = :a AND b > :b), DESC flipping the inequality — so NULL-aware after() #394 can extend that function. Datetime query binds use pydantic JSON (…Z for UTC) so SQLite TEXT equality matches INSERT.

Test plan

• Build-time: no PK in order keys; PK-less model; nullable order key; wrong arity; None in a slot; after+offset; update/delete reject after; count() omits after
• e2e sqlite: order_by(updated_at).order_by(id).after((ts, id)).limit(n) returns the next n exclusive, in declared order; empty page past the end; after(row) equals after(position_of(row))
• Golden vector query_user_after_v13 plus all query vectors bumped to ir_version 13
• Rust: exclusive stepwise compare ASC and DESC on two NOT NULL keys; QueryIR accepts v13
• Postgres e2e locally (pytest-postgresql could not start; no FERRO_POSTGRES_URL)

Closes #393

Made with Cursor