Build Systems

Create ecs/system-database/system-database.ts: Database.Plugin.create({ extends: Database.Plugin.combine(ComputedDatabase.plugin, scheduler), systems }) where the systems map is declared inline (see features/ecs/systems.md — inline is required for db to be typed and for system-name inference; a systems/ folder is optional, only for extracted per-frame body helpers).

• A system is a SystemDeclaration: { create, schedule?: { before?, after?, during? } }. create(db) runs once (capture queries / index handles / closures); the returned function advances the world one tick. Return void for an init-only system (seeding entities).
• The step math lives in data/; each system reads store rows, calls the pure data/ step, writes back — via db.store (writable) for hot per-row work, or db.transactions.* for discrete events. Iterate archetypes per archetypes.md (tail→head when destroying rows).
• Order under schedule (mirror the data/ step's internal sequence); drive the loop by combining scheduler (rAF), gated by the schedulerState resource.

Only for real-time features (games, sims) — turn-based features skip this phase entirely. Comes after build-computed. The how is in the auto-loading features/ecs/systems.md rule.