Pyrite only: drop the Renode/C# backend; the sim binary is the simulator, Python orchestrates #56

The C# engine is halted and its release product (cli/) is retired, yet simantic.Sim, simantic.run() and the pytest plugin still default to backend="renode" (pythonnet + Simantic.Core), which can no longer even be fetched. The Rust paths are a second runner beside the sim binary: run_scenario builds its own request and rejects parts: (no peers), Sim(backend="rust") refuses multi-machine scenarios. Users who need peers, two machines, bridges or --record get either a dead engine or NotSupported.

Direction: the sim binary is the source of truth; pippy is a thin orchestration layer.

• Remove the renode backend, the pythonnet/.NET loader, install_engine, the pythonnet dependency, --sim-backend.
• run() → sim <MCU> run --elf …; run_scenario() and pytest test.yaml items → sim run --scenario … --record DIR, read the bundle.
• Sim stays on the in-process pyrite engine (single machine, stepping); documented as such.
• README/docs: the binary is the simulator; Python orchestrates.

Taking this up: Claude session, branch agent/pyrite-only-5b8e, 2026-10-27.