add-system

name	add-system
description	Add a new server simulation system (combat-like, movement-like, spawning, regen, etc.) to the 20Hz tick. Use when a request needs new server-side behavior beyond what content manifests can express.

Add a server system

First decide the tier: if this is a reusable/optional behavior, write it as a plugin system (/add-plugin, registerSystem) instead of a core system — same shape, no core edits.

For a core system:

1. Create packages/server/src/systems/<name>.ts exporting a pure function over state: export function <name>System(state: GameState, dt: number): void. Use movement.ts or stamina.ts as the template. No I/O, no timers — the tick drives everything.
2. Register it in the tick: packages/server/src/rooms/GameRoom.ts → setSimulationInterval body, wrapped in a span so it shows in F3 + /api/perf: perfTracker.span("<name>", () => <name>System(this.state, dt)); Insert in dependency order (e.g. after movement if it reads positions).
3. New synced fields? Add @type decorators in packages/shared/src/schema/*, then pnpm build:shared and a client hard reload.
4. Tunables go in packages/shared/src/constants.ts; live-tweakable knobs additionally in systems/devTuning.ts.
5. Add a characterization test packages/server/src/systems/<name>.test.ts (hand-built GameState, fixed dt — see combat.test.ts). Run pnpm test.
6. Verify cost: pnpm bench — the new span appears as tag:<name>; keep idle cost ≈0ms.