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/ScenarioSnapshot.hpp

@@ -0,0 +1,54 @@
+#pragma once
+#include "FluidSim/EngineConfig.hpp"
+#include "FluidSim/Fluid.hpp"
+#include "FluidSim/OutdoorAudio.hpp"
+#include "FluidSim/Silencer.hpp"
+#include <array>
+#include <cstdint>
+#include <string>
+#include <vector>
+
+namespace fluidsim::port {
+struct ScenarioSnapshot {
+    std::string example_name;
+    EngineLayout layout{EngineLayout::Inline};
+    float bank_angle{};
+    bool v8_headers{};
+    bool staged_headers{}, dual_exhaust{};
+    PipeSolver pipe_solver{PipeSolver::Rusanov};
+    int cylinder_count{2};
+    float rev_limit_rpm{10000};
+    std::array<float, 12> cylinder_angles{};
+    std::array<float, 12> cylinder_pressures{}, piston_fractions{}, intake_areas{}, exhaust_areas{};
+    std::uint64_t solver_substeps{}, boundary_iterations{}, pipe_positivity_repairs{};
+    float rpm{}, speed_kmh{}, kinetic_energy{}, average_power{}, average_torque{};
+    float cylinder_pressure1{}, cylinder_pressure2{};
+    float cylinder_temperature1{}, cylinder_temperature2{};
+    float cylinder_piston_fraction1{}, cylinder_piston_fraction2{};
+    float valve1_intake_area{}, valve1_exhaust_area{}, valve2_intake_area{}, valve2_exhaust_area{};
+    float intake_pressure{}, exhaust_pressure{}, second_bank_pressure{};
+    std::array<float,2> crossover_pressures{};
+    float throttle{}, pedal{}, target_afr{}, clutch{}, intake_flow{}, exhaust_flow{};
+    bool idle_control{}, fuel_cut{}, ignition_cut{}, two_stroke{}, reed_open{};
+    bool racing_exhaust{};
+    bool silencers{}, h_pipe{};
+    ExhaustConfig exhaust_config{};
+    float crankcase_pressure{};
+    OutdoorScene audio_scene{OutdoorScene::Wall};
+    int gear{};
+    bool rev_limit1{}, rev_limit2{};
+    std::array<std::vector<GasState>, 32> pipe_states;
+    std::array<std::vector<float>, 32> pipe_areas, pipe_lengths;
+    void capture_pipes(const std::vector<Pipe1D>& pipes) {
+        for (std::size_t k = 0; k < pipes.size(); ++k) {
+            pipe_positivity_repairs += pipes[k].positivity_repairs;
+            pipe_areas[k] = pipes[k].cell_areas;
+            pipe_lengths[k] = pipes[k].cell_lengths;
+            pipe_states[k].reserve(pipes[k].cells.size());
+            for (std::size_t i = 0; i < pipes[k].cells.size(); ++i)
+                pipe_states[k].push_back(pipes[k].state(static_cast<int>(i)));
+        }
+    }
+};
+
+}