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.

third_party/tomlplusplus/README.md

@@ -0,0 +1,8 @@
+# toml++
+
+Vendored single-header parser, version **3.4.0**, from
+[marzer/tomlplusplus](https://github.com/marzer/tomlplusplus/tree/v3.4.0).
+
+`toml.hpp` is the upstream amalgamated header. Its MIT license is retained in
+`LICENSE`. Only `src/engine_config.cpp` includes the third-party parser; public
+configuration headers use project types.