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/Desktop/DesktopUi.hpp

@@ -1,5 +1,5 @@
 #pragma once
-#include "FluidSim/Scenario.hpp"
+#include "FluidSim/ScenarioSnapshot.hpp"
 #include "FluidSim/DynoCurve.hpp"
 #include "FluidSim/Desktop/PedalControl.hpp"
 #include "FluidSim/Desktop/EngineRenderer3D.hpp"
@@ -22,6 +22,7 @@ public:
     void nudge_pedal(float amount) {pedal_.set(pedal_.value()+amount);}
     void update_pedal(float dt,bool raise,bool lower) {if(!pedal_drag_)pedal_.advance(dt,raise,lower);}
     bool handle_event(const sf::Event&,sf::Vector2u);
+    AudioListener listener(const ScenarioSnapshot&,sf::Vector2u);
     void draw(sf::RenderWindow&,const sf::Font*,const ScenarioSnapshot&,const PlaybackInfo&);
 private:
     DynoCurve dyno_;