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/EngineConfig.hpp

@@ -0,0 +1,53 @@
+#pragma once
+#include "FluidSim/Engine.hpp"
+#include "FluidSim/Ecu.hpp"
+#include "FluidSim/Vehicle.hpp"
+#include "FluidSim/Carburetor.hpp"
+#include <string>
+#include <vector>
+
+namespace fluidsim::port {
+enum class ScenarioKind { Twin, TwoStroke, SaabB205, AmericanV8 };
+enum class FourStrokePreset { Twin, SaabB205, AmericanV8 };
+enum class EngineLayout { Inline, V, Boxer };
+enum class ExhaustTopology { Collector, Staged, AmericanV8 };
+
+struct PipeConfig {
+    std::string id;
+    float diameter{}, length{}; // metres
+    int cells{}, count{1};
+    bool silenced{};
+};
+
+// Parsed calibration only. Runtime state belongs to the scenario components.
+// Dimensional fields use SI units after loading.
+struct EngineConfig {
+    int cylinders{};
+    EngineLayout layout{EngineLayout::Inline};
+    ExhaustTopology topology{ExhaustTopology::Collector};
+    float bank_angle{60};
+    CylinderConfig cylinder;
+    CrankshaftConfig crankshaft;
+    EcuConfig ecu;
+    VehicleConfig vehicle;
+    CarburetorConfig fuel;
+    bool carbureted{}, initialize_phased_charge{true};
+    float initial_rpm{}, starter_torque{}, maximum_load_torque{};
+    float plenum_volume{}, collector_volume{}, throttle_diameter{}, pipe_damping{};
+    int minimum_substeps{1};
+    std::vector<float> phase_degrees;
+    std::vector<PipeConfig> pipes;
+
+    bool has_silenced_exhaust() const { for(const auto& p:pipes)if(p.silenced)return true;return false; }
+    bool american_headers() const { return topology==ExhaustTopology::AmericanV8; }
+    bool staged_headers() const { return topology==ExhaustTopology::Staged; }
+    bool dual_outlets() const { return american_headers() || (staged_headers() && layout==EngineLayout::V); }
+    int header_groups() const { return cylinders==3?1:2; }
+};
+
+struct JunctionConfig {
+    bool h_pipe{true};
+    float junction_volume{50e-6f};
+};
+void validate_engine_config(const EngineConfig& config,const std::string& context="engine config");
+}