Write a SpacetimeDB backend module in TypeScript that models a minimal Entity Component System with joins.

TABLES
- entity
  - Fields:
    - id: number (i32, primary key)

- position
  - Fields:
    - entityId: number (i32, primary key)
    - x: number (i32)
    - y: number (i32)

- velocity
  - Fields:
    - entityId: number (i32, primary key)
    - vx: number (i32)
    - vy: number (i32)

- nextPosition
  - Fields:
    - entityId: number (i32, primary key)
    - x: number (i32)
    - y: number (i32)

REDUCERS
- seed: insert 2 entities with positions and velocities:
  - Entity 1: position(0,0), velocity(1,0)
  - Entity 2: position(10,0), velocity(-2,3)

- step: for each position, find velocity, compute next position (x+vx, y+vy), upsert to nextPosition
