Generic synchronous interpreter for the pure Normandy.Agents.Turn core.

Drives a turn to a stop by feeding :start into Turn.step/2, performing each returned effect via an injected %Handlers{} set, and feeding the resulting event back into step/2 until the turn reaches a terminal effect. The driver owns FSM stepping and acc threading; the handlers own all side effects (LLM calls, tool dispatch, memory, guards, telemetry) and return the updated acc.

acc is opaque to the driver — the production shells pass the running %BaseAgentConfig{} (the memory accumulator). This lets one driver serve the non-streaming and streaming production paths (and future shells) with different handler sets, the same way Turn.Inline serves the test/library path.

By design, step/2 always places the single blocking/terminal effect last in its effect list, so the driver performs the leading :emit_event / :append_message effects in order, then acts on the terminal one.