Build Systems

Skip if this feature doesn't contain or require systems. Most games that use a canvas will use systems but most applications will not.

Create services/main-service/system-database/system-database.ts: Database.Plugin.create({ extends: Database.Plugin.combine(<currentTop>.plugin, scheduler), systems }) — combine scheduler with the feature's current top main-service layer (ActionDatabase / ServiceDatabase / ComputedDatabase, whichever it built), never a hardcoded ComputedDatabase, so systems and any services/actions compose into the one MainService. The systems map is declared inline (see features/services/main-service/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).
• 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/services/main-service/systems.md rule.