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.

src/main.cpp

@@ -1,4 +1,5 @@
 #include "FluidSim/Scenario.hpp"
+#include "FluidSim/ScenarioCatalog.hpp"
 #include "FluidSim/HelmholtzScenario.hpp"
 #include "FluidSim/PulseTestScenario.hpp"
 #include "FluidSim/TwoStrokeScenario.hpp"
@@ -10,12 +11,12 @@
 #include <iostream>
 #include <string>
 
-int main(int argc, char** argv) {
+int main(int argc, char** argv) try {
     using namespace fluidsim::port;
     float duration_seconds = 1.0f;
     float throttle = 0.35f;
     std::string scenario_name = "twin";
-    TwoStrokeConfig two_stroke_config;
+    auto two_stroke_config=scenario_example("yz-racing").two;
     PipeSolver pipe_solver=PipeSolver::Rusanov;
     for (int i = 1; i + 1 < argc; ++i) {
         const std::string option = argv[i];
@@ -107,4 +108,7 @@ int main(int argc, char** argv) {
         return 2;
     }
     return 0;
+} catch(const std::exception& error) {
+    std::cerr<<error.what()<<'\n';
+    return 2;
 }