FR: @agg: collect with @of optional — whole-object rollup (the designated origin.collection re-entry shape)

[dmealing]

Additive. Make @of optional on origin.aggregate @agg: collect, where absent means a
whole-object rollup: collect the related rows as an array of the field's declared
@objectRef value-object, rather than an array of one scalar column.

{ "field.object": {
    "name": "supplierBriefs",
    "isArray": true,
    "@objectRef": "acme::common::SupplierBrief",
    "children": [
      { "origin.aggregate": { "@agg": "collect", "@via": "acme::catalog::Product.suppliers" } }
    ]
}}

Today that requires @of, so only a single scalar column can be collected — an array of
values, never an array of objects.

Why now / why this shape

This is the designated re-entry shape recorded in FR-037 R2 when origin.collection was
retired to reserved-not-registered. origin.collection expressed exactly this idea and
never emitted anything: there is no collection column kind, and codegen-ts/src,
migrate-ts/src and runtime-ts/src contain zero references to it. Its last real consumer,
the payload-VO typing edge, was deleted in 0.20.16 (#270) for being actively wrong. So the
capability has been declarable for a long time and never worked.

R2 deliberately did not activate the fold-in at retirement time, on the ADR-0007 Amendment 2
bar — a member enters the registry only when a shipping consumer dispatches on it. This
issue is that bar being met: build the consumer, then the vocabulary earns its place.

Parent-with-children-inline is one of the most common read-model shapes, and hand-writing
that view is precisely what a declared projection exists to avoid.

Why it is cheap on the vocabulary axis

@of is already required: false in expected-registry.json. The "collect requires
@of" constraint lives in validation, not the registry:
server/typescript/packages/metadata/src/loader/validation-passes.ts →
// --- count/sum/avg/min/max/collect: @of REQUIRED ---, where collect is grouped with the
scalar aggregates.

So this needs no registry change and is additive — it relaxes a rule, making
previously-invalid metadata valid. No metamodelVersion move, no breaking slot.

Design

1. Loader — all four ports. Split collect out of the @of-required gate.

• @of ABSENT → require the carrying field to be field.object with @objectRef
  (isArray: true is already enforced for collect). The element type is
  declared-authoritative (Prelude to #210: retire redundant payload origin-typing (Kotlin, Python) and close Java's KNOWN_GAPS entry as moot #270): type from the declared @objectRef, never derived from
  the @via relationship's target entity. @objectRef must target an object.value, which
  is already a loader rule from Retire assembly origins on object.value (one spelling of derived read shape) — RULED: batch into the pre-1.0 consolidation #210.
• @of PRESENT → unchanged, including today's element-type-preserving check.

2. Codegen — TypeScript only. Projection view DDL is TS-owned (ADR-0015).

• projection/extract-view-spec.ts — the AGG_COLLECT branch currently sits inside @of
  resolution. Add a whole-object arm that resolves the target entity from @via and emits a
  new column kind (e.g. collectObjectAgg) carrying the declared value-object's field list
  with their resolved physical column names.
• projection/view-ddl-emit.ts — mirror the existing scalar lowering, including its
  FILTER (WHERE <joined pk> IS NOT NULL) empty-set guard and @orderBy / @distinct
  handling:
  • postgres: COALESCE(json_agg(json_build_object('f', a.c, …) ORDER BY …) FILTER (WHERE … IS NOT NULL), '[]')
  • sqlite: COALESCE(json_group_array(json_object('f', a.c, …)) FILTER (…), json_array())
• The projection column is json/jsonb typed as an array of the declared value-object.

Acceptance

• Loader: @of-absent collect on a field.object @objectRef array loads in all five ports;
  the same without @objectRef, or targeting a non-value, errors. Cross-port conformance
  fixtures both ways.
• No-churn: a collect with @of emits byte-identical SQL to today.
• A view-lifecycle round-trip against a real engine, both dialects — emit → apply →
  introspect → re-diff must be empty, and read the rows back to confirm the array-of-objects
  shape. Golden-comparing new SQL is how the migrate defects got through before; new DDL does
  not ship on a snapshot alone.
• flattened-kitchen-sink's supplierBriefs field, dropped when origin.collection
  retired, comes back expressed this way.

Not in scope

Non-RDB lowerings — that is #211's capability matrix.