SU.858 Material-handling layout, simulation and PLC emulation (conveyors, stations, AGVs)

Asked: emulation and simulation software for material handling that costs about $40,000 a year (#858, Oct 20; 43 votes, the most-voted ask).

Do (docs/0.8-PART2.md § SU.858): a ModelCraft add-on `flow`: a catalogue of conveyors, merges, diverters, stations, buffers, sources/sinks, AGVs and sensors with ports; snap a layout in 3D; a deterministic discrete-event simulation with accumulation and routing rules; playback in 3D, statistics, CSV. Later, Studio only: PLC emulation over Modbus TCP or OPC UA (owner picks the first). No physics; loads are kinematic.

PLC mode (2026-11-06): the panel's PLC mode chooses Soft or Real, saved with the model. Soft runs the Digital twin's soft PLC (ST or PLCopen XML) on the layout with no hardware, in the browser or Studio, repeatable to the byte. Real talks Modbus TCP to a PLC through Studio (plc.rs client commands) with an address map editor (tag, type, table, address, scale); in a browser tab it is off with the reason. Simulation-only safety line; the PLC's address is never saved in the file. Tested: Modbus spec known answers, loopback against a JS Modbus server, the real /lab page. Needs the 0.9 Studio build (Rust core tested alone, Tauri wrappers not compiled here). Real PLC check: community row SU.858-HAND.