ecs: name archetype rows without a cast + flatten combine's IntersectAll

[krisnye]

Description

Two related type-system improvements in @adobe/data (packages/data/src/ecs).

Part 1 — name archetype rows without a cast (Database.Archetype.RowOf)

db.archetypes.<Name> is a ReadonlyArchetype<Row> whose Row is derived from the declared columns. Consumers exposing an archetype under a hand-authored service interface had to launder it through as unknown as at every public boundary (and per-entity in db.observe.entity(id, archetype)).

We considered carrying an explicit "brand" type through inference, but confirmed branding adds no type checking — it only lets you hide internal columns behind a narrower public row. The full structural row is fine here, so we drop branding entirely. The cast only ever arose because a service type re-declared archetype rows by hand; a service type derived from the plugin (Database.Plugin.ToDatabase<typeof plugin>) already has the same row type, so db.archetypes assigns with no cast.

• Add Database.Archetype.RowOf<S, K> (reusing the existing FromArchetype) so a row can be named without re-spelling its columns.
• No runtime change; no change to ArchetypeComponents, the store mapped type, or create-plugin.
• New archetype-row.type-test.ts proves the no-cast assignment, RowOf equality, ReadonlyArchetype<RowOf<…>>, and the observe.entity row flow. README note steers consumers to derive, not re-declare or cast.

type MainService = Database.Plugin.ToDatabase<typeof plugin>;
type Track = Database.Archetype.RowOf<MainService, "Track">;
const t: ReadonlyArchetype<Track> = db.archetypes.Track; // no cast

Part 2 — flatten combine's IntersectAll

combine-plugins.ts intersected each of the 9 plugin buckets with a linearly-recursive, Simplify-wrapped IntersectAll<T> = Simplify<H & IntersectAll<R>>. The per-level Simplify forced eager re-materialization of the intersection (×9 buckets — the bulk of the instantiation count).

Reformulated using the already-shipped non-recursive UnionToIntersection<T[number]> (empty-tuple guarded), and UnionAll → X[number] for the systems union.

• ~29% fewer instantiations (296,595 → 210,305) on a 24-plugin single combine (measured locally; everything else constant).
• Behavior-preserving: merged buckets are object maps that intersect soundly; the === identity conflict guard and merge order are value-level and unchanged. systems stays a real union.
• New combine-plugins.type-test.ts guards correct merged-type resolution over a wide single combine.

Verification

• tsc -b clean; eslint clean on changed files.
• All 2585 package tests pass (one timing-ratio perf test flaked under parallel load; passes in isolation — Part 2 is type-only and cannot affect runtime).
• extends typeperf baseline unchanged (that path goes through CreatePluginResult, not combine).

Related PRs

Follows #120 (thread index declarations into the computed-factory db type) and #117 (index API).