Write a SpacetimeDB backend module in C# that models a minimal ECS and computes next positions.

TABLES
- Entity
  - Struct: Entity
  - Fields:
    - Id: ulong (primary key, auto-increment)

- Position
  - Struct: Position
  - Fields:
    - EntityId: int (primary key) — NOT auto-increment, this is a foreign key to Entity
    - X: int
    - Y: int

- Velocity
  - Struct: Velocity
  - Fields:
    - EntityId: int (primary key) — NOT auto-increment, this is a foreign key to Entity
    - VX: int
    - VY: int

- NextPosition
  - Struct: NextPosition
  - Fields:
    - EntityId: int (primary key) — NOT auto-increment, this is a foreign key to Entity
    - X: int
    - Y: int

REDUCERS
- Seed: insert two entities with positions and velocities (entity IDs are auto-assigned):
  - (Pos=(0,0), Vel=(1,0))
  - (Pos=(10,0), Vel=(-2,3))
- Step: for each Position, find Velocity by EntityId; compute (X+VX, Y+VY) and upsert into NextPosition by EntityId
