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.

CMakePresets.json

@@ -39,7 +39,6 @@
                 "CMAKE_BUILD_TYPE": "RelWithDebInfo",
                 "CMAKE_C_COMPILER": "$penv{LOCALAPPDATA}/FluidSimTools/mingw64/bin/gcc.exe",
                 "CMAKE_CXX_COMPILER": "$penv{LOCALAPPDATA}/FluidSimTools/mingw64/bin/g++.exe",
-                "CMAKE_MAKE_PROGRAM": "$penv{LOCALAPPDATA}/FluidSimTools/mingw64/bin/ninja.exe",
                 "FLUIDSIM_BUILD_DESKTOP": "ON",
                 "BUILD_TESTING": "ON"
             }