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.

CMakeLists.txt

@@ -5,6 +5,7 @@ option(FLUIDSIM_BUILD_DESKTOP "Build the SFML windowed simulator with keyboard c
 set(FLUIDSIM_FONT_FILE "" CACHE FILEPATH "Optional font file copied beside the desktop executable")
 
 add_library(fluidsim_core
+    src/engine_config.cpp
     src/carburetor.cpp
     src/ecu.cpp
     src/engine.cpp
@@ -17,11 +18,15 @@ add_library(fluidsim_core
     src/outdoor_audio.cpp
     src/outdoor_exhaust_reverb.cpp
     src/scenario.cpp
+    src/scenario_snapshot.cpp
+    src/scenario_geometry.cpp
     src/solver.cpp
     src/vehicle.cpp
     src/simulation_ring_buffer.cpp
 )
 target_include_directories(fluidsim_core PUBLIC include)
+target_include_directories(fluidsim_core SYSTEM PRIVATE third_party/tomlplusplus)
+target_compile_definitions(fluidsim_core PRIVATE FLUIDSIM_ENGINE_SOURCE_DIR="${CMAKE_CURRENT_SOURCE_DIR}/engines")
 target_compile_features(fluidsim_core PUBLIC cxx_std_20)
 option(FLUIDSIM_ENABLE_SIMD "Allow compiler SIMD in fluid kernels without fast-math" ON)
 if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang")
@@ -109,6 +114,9 @@ endif()
 
 include(CTest)
 if(BUILD_TESTING)
+    add_executable(fluidsim_engine_config_tests tests/engine_config_tests.cpp)
+    target_link_libraries(fluidsim_engine_config_tests PRIVATE fluidsim_core)
+    add_test(NAME fluidsim_engine_config_tests COMMAND fluidsim_engine_config_tests)
     add_executable(fluidsim_scenario_examples tests/scenario_examples.cpp)
     target_link_libraries(fluidsim_scenario_examples PRIVATE fluidsim_core)
     add_test(NAME fluidsim_scenario_examples COMMAND fluidsim_scenario_examples)
@@ -167,4 +175,24 @@ if(BUILD_TESTING)
     add_test(NAME fluidsim_b205_regression COMMAND fluidsim_b205_regression)
 endif()
 
+add_executable(fluidsim_carb_calibration tools/carb_calibration.cpp)
+target_link_libraries(fluidsim_carb_calibration PRIVATE fluidsim_core)
+if(BUILD_TESTING)
+    add_test(NAME fluidsim_carb_calibration COMMAND fluidsim_carb_calibration --check)
+endif()
+
 include(cmake/MinGWRuntime.cmake)
+
+# Keep definitions beside every built executable, including tests and CLI tools.
+add_custom_target(fluidsim_engine_assets ALL
+    COMMAND ${CMAKE_COMMAND}
+        "-DSOURCE=${CMAKE_CURRENT_SOURCE_DIR}/engines"
+        "-DDESTINATION=${CMAKE_CURRENT_BINARY_DIR}/engines"
+        -P "${CMAKE_CURRENT_SOURCE_DIR}/cmake/CopyEngineConfigs.cmake")
+get_property(fluidsim_targets DIRECTORY PROPERTY BUILDSYSTEM_TARGETS)
+foreach(target IN LISTS fluidsim_targets)
+    get_target_property(target_type ${target} TYPE)
+    if(target_type STREQUAL "EXECUTABLE")
+        add_dependencies(${target} fluidsim_engine_assets)
+    endif()
+endforeach()