`src/engine/bare/model_switch.rs` — `ModelSwitch`

The atomic mid-session model change: a builder returned by agent.switch_model(name), executed by .apply().

Key items

• switch_model(&mut self, model) → ModelSwitch — holds &mut BareLoop, deliberately not Clone.
• .with_context_window(tokens) — set it when the new model’s window differs.
• .apply() — validate name → poison preflight → client.set_model (false = abort, nothing changed) → optional window update → breaker reset + original-model update → on_model_switched.

Behavior notes

• All-or-nothing: a rejection at any step leaves the loop exactly as it was.
• Requires client support (ApiClient::set_model) — all shipped clients have it; the trait default rejects.
• Resets the fallback breaker because old-model failure counts are meaningless for the new model.
• Not gated by “configure before run” — mid-session use is the design (still: prefer between runs over during).

Deep dive: Model switch.