Load engines from TOML and improve scenario audio and carb tuning

Migrate 32 engine definitions into validated startup-loaded TOML files and separate configuration and snapshots from scenario assembly. Add camera-based stereo propagation from shared pipe geometry, preserve exhaust character, tune carburetor mixtures and idle, use ignition cuts for carb engines, and support returning to the scenario menu.

Validation: full Windows build and all 14 CTest checks passed.

include/FluidSim/TwoStrokeScenario.hpp

@@ -19,7 +19,7 @@ namespace fluidsim::port {
 struct TwoStrokeSnapshot {
     float rpm{}, cylinder_pressure{}, crankcase_pressure{}, intake_pressure{};
     float throttle{}, transfer_area{}, exhaust_area{};
-    bool reed_open{}, fuel_cut{};
+    bool reed_open{}, fuel_cut{}, ignition_cut{};
     float pedal{};
     OutdoorScene audio_scene{OutdoorScene::Wall};
 };
@@ -28,6 +28,7 @@ struct TwoStrokeSnapshot {
 // the plenum / reed / crankcase / transfer / cylinder / exhaust topology.
 class TwoStrokeScenario : public MotorcycleTest {
 public:
+    void set_listener(AudioListener listener) { audio_.set_listener(listener); }
     explicit TwoStrokeScenario(int sample_rate = 44100, TwoStrokeConfig config = {});
     AudioFrame process();
     void set_throttle(float value);