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

TABLES
- Entity
  - Struct: Entity
  - Fields:
    - Id: int (primary key)

- Position
  - Struct: Position
  - Fields:
    - EntityId: int (primary key)
    - X: int
    - Y: int

- Velocity
  - Struct: Velocity
  - Fields:
    - EntityId: int (primary key)
    - VX: int
    - VY: int

- NextPosition
  - Struct: NextPosition
  - Fields:
    - EntityId: int (primary key)
    - X: int
    - Y: int

REDUCERS
- Seed: insert two entities with positions and velocities:
  - (Id=1, Pos=(0,0), Vel=(1,0))
  - (Id=2, 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
