[P3][lens] Relational lens capability arc: nullable columns, unique constraints, order-by, aggregation

[cevheri]

Summary

Roadmap umbrella (DESIGN.md section 6.2 honest deferrals for the relational lens): nullable/optional columns, unique constraints, order-by beyond primary-key order, aggregation/group-by, outer and non-equi joins, foreign keys. Each is deliberately out of v1; this issue tracks scoping them for v2 — individually, against the comprehension budget, and explicitly NOT as a step toward SQL text (still rejected by DESIGN.md section 2).

Suggested order of scoping

1. Nullable/optional columns — the most-requested ergonomic gap; touches validateRow and the catalog schema shape (a schema change, so it interacts with the no-migration rule).
2. Unique constraints — natural sibling of the secondary-index work (an index IS the enforcement mechanism).
3. order-by — trivial for indexed fields, O(n log n) materialized sort otherwise; must be documented honestly either way.
4. Aggregation/group-by, outer/non-equi joins, foreign keys — only if real usage asks for them.

Constraints

• Schema evolution is its own roadmap item (see the schema-migration issue); nullable columns must not smuggle migration in through the back door.
• Kernel unchanged; catalog entries stay the single source of cold-open truth for Studio.
• Split into per-capability issues before any implementation; changeset per shipped capability.