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.

cmake/CopyEngineConfigs.cmake

@@ -0,0 +1,14 @@
+file(MAKE_DIRECTORY "${DESTINATION}")
+file(GLOB definitions "${SOURCE}/*.toml")
+foreach(definition IN LISTS definitions)
+    get_filename_component(name "${definition}" NAME)
+    configure_file("${definition}" "${DESTINATION}/${name}" COPYONLY)
+endforeach()
+# Remove only stale definitions in the build's asset directory.
+file(GLOB old_definitions "${DESTINATION}/*.toml")
+foreach(definition IN LISTS old_definitions)
+    get_filename_component(name "${definition}" NAME)
+    if(NOT EXISTS "${SOURCE}/${name}")
+        file(REMOVE "${definition}")
+    endif()
+endforeach()