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/Vehicle.hpp

@@ -5,16 +5,19 @@
 namespace fluidsim::port {
 
 // Longitudinal vehicle model and the friction clutch coupling it to the crank.
-class Vehicle {
-public:
-    int current_gear{};
+struct VehicleConfig {
     std::vector<float> gear_ratios{4.1f, 2.7f, 2.0f, 1.55f, 1.25f, 0.98f};
     float final_drive_ratio{2.7f}, primary_reduction{1.8f};
-    float clutch_input{1.0f}, clutch_capacity{80.0f}, clutch_engagement_rate{8.0f};
+    float clutch_capacity{80.0f}, clutch_engagement_rate{8.0f};
     float mass{160.0f}, wheel_radius{0.32f}, rotational_inertia{1.2f};
     float rolling_resistance{0.015f}, drag_coefficient{0.7f}, frontal_area{0.65f};
     float air_density{1.225f}, gravity{9.80665f};
-    float speed{}, engagement{};
+};
+
+class Vehicle : public VehicleConfig {
+public:
+    int current_gear{};
+    float clutch_input{1.0f}, speed{}, engagement{};
 
     float total_gear_ratio() const;
     float kinetic_energy() const;