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

TABLES
- entities
  - Struct: Entity
  - Fields:
    - id: i32 (primary key)

- positions
  - Struct: Position
  - Fields:
    - entity_id: i32 (primary key)
    - x: i32
    - y: i32

- velocities
  - Struct: Velocity
  - Fields:
    - entity_id: i32 (primary key)
    - vx: i32
    - vy: i32

- next_positions
  - Struct: NextPosition
  - Fields:
    - entity_id: i32 (primary key)
    - x: i32
    - y: i32

REDUCERS
- seed: insert the two entities with positions/velocities:
  - (id=1, pos=(0,0), vel=(1,0))
  - (id=2, pos=(10,0), vel=(-2,3))
- step: for each position, find velocity by entity_id; compute (x+vx, y+vy) and upsert into next_positions by entity_id
