Fork for what-ifs

# A Kinematic mover at the origin, 1 unit/s along +x.
curl -s -X POST http://localhost:3917/spawn \
  -H 'Content-Type: application/json' \
  -d '{"position":[0,1,0],"velocity":{"linear":[1,0,0]},"physics_body":"Kinematic","health":100,"team":1}'
# { "entity_id": 1, "entity_generation": 0 }

# Advance the real run a little so "now" is a non-trivial tick.
curl -s -X POST http://localhost:3917/step \
  -H 'Content-Type: application/json' -d '{"ticks":30}'
# { "ticks_advanced": 30, "new_tick": 30, "entity_count": 2 }

curl -s -X POST http://localhost:3917/fork \
  -H 'Content-Type: application/json' -d '{"fork_id":"reach-x3"}'
# { "ok": true, "fork_id": "reach-x3", "parent_tick": 30 }

# Advance ONLY the fork by 200 ticks.
curl -s -X POST http://localhost:3917/fork/reach-x3/step \
  -H 'Content-Type: application/json' -d '{"ticks":200}'
# { "ok": true, "fork_id": "reach-x3", "start_tick": 30, "new_tick": 230, "ticks_advanced": 200, "entity_count": 2 }

# Read the fork's state and check the mover's x.
curl -s http://localhost:3917/fork/reach-x3/observe
# → the mover (id 1) is at "position": [3.8333303928375244, 1.0, 0.0] at tick 230

# Main world is still at tick 30 — the what-if changed nothing.
curl -s -X POST http://localhost:3917/observe        # → "tick": 30

# Discard the fork.
curl -s -X DELETE http://localhost:3917/fork/reach-x3
# { "ok": true, "deleted": "reach-x3" }