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/AudioProcessing.hpp

@@ -4,6 +4,7 @@ namespace fluidsim::port {
 
 struct AudioFrame { float left{}, right{}; };
 struct Vec3 { float x{}, y{}, z{}; };
+struct AudioListener { Vec3 position{}, right{1,0,0}; };
 
 // Simple flow-derivative source used by the Helmholtz and pulse scenarios.
 class SoundProcessor {