build-core-database

Legacy feature (has `data/state/` or `services/main-service/`)? Extend it in place per the legacy rules; don't migrate unless asked. Create: - `ecs/core/archetypes.ts` — `Database.archetypes(components, { … })`, spreading each `data/entities` tuple and adding implementation-only columns (never a `nonPersistent` column without a real default). - `ecs/core/core-database.ts` — `Database.Plugin.create({ extends?, components?, resources?, archetypes? })` from the `data/` barrels, exporting the `CoreDatabase` namespace. A feature built on another extends that feature's core. - `ecs/main-service.ts` — `export { CoreDatabase as MainService }`, moved up as layers are added. - `ecs/conformance/` — `projection.ts`, `implementation.ts` (`Conformance.implementation(spec, …)`) and `conformance.test.ts` (`Conformance.checkFeature`). Gate: the samples round-trip through the projection. Until the later layers exist, `checkFeature` lists each spec op without an implementation by name; that list is the work remaining. The how is in the auto-loading `features/ecs/index.md`, `core.md` and `conformance.md` rules.