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.

include/FluidSim/ScenarioGeometry.hpp

@@ -0,0 +1,16 @@
+#pragma once
+#include "FluidSim/AudioProcessing.hpp"
+#include <array>
+
+namespace fluidsim::port {
+struct ScenarioSnapshot;
+// Fractions refer to simulated pipe length; positions are world metres.
+// Both the pipe mesh and its acoustic mouths use these same routes.
+struct PipeRoute {
+    std::array<Vec3, 5> points{};
+    std::array<float, 5> fractions{};
+    unsigned count{};
+};
+std::array<PipeRoute, 32> scenario_pipe_routes(const ScenarioSnapshot& state);
+std::array<Vec3, 3> scenario_sound_sources(const ScenarioSnapshot& state);
+}