Solo. Play Minecraft (primary, Fabric). Garry's Mod (companion) runs in a side window; every block you break falls there as a physics prop (HL2 models from the player's own GMod copy, not shipped). Bridge: Minecraft appends JSON lines to {gmod}/garrysmod/data/two_worlds/events.jsonl; the GMod addon polls it. Design lives in sheets/*.json. Run tools/preflight.py, then tools/gen_lua.py, then build fabric-mod. NOT TESTED