Add American V8 preset, carb fuel transport, and adaptive desktop audio

include/FluidSim/Carburetor.hpp

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+#pragma once
+
+#include <array>
+#include <cstddef>
+
+namespace fluidsim::port {
+
+// Lumped slide-carburetor model for the crankcase-scavenged two-stroke. The
+// jets use liquid-orifice flow from a float bowl into the measured venturi
+// depression. Delivered fuel is injected into the downstream fluid volume and
+// is then transported as a conserved species; this class never doses a cylinder.
+class Carburetor {
+public:
+    float fuel_density{740.0f};
+    float pilot_jet_diameter{0.00040f}, main_jet_diameter{0.00150f};
+    float needle_jet_diameter{0.00190f};
+    float pilot_discharge_coefficient{0.62f}, main_discharge_coefficient{0.72f};
+    float bowl_head_pressure{};
+    float film_fraction{0.28f}, film_time_constant{0.11f};
+    float bowl_capacity{0.008f}, bowl_refill_rate{0.0025f}; // kg, kg/s
+    float overrun_rpm{4200.0f}, overrun_exit_rpm{3300.0f};
+
+    explicit Carburetor(float throat_diameter = 0.0381f);
+
+    // air_mass_flow is positive from atmosphere toward the engine.
+    void step(float dt, float pedal, float slide, float rpm, float air_mass_flow,
+              float air_density, float upstream_pressure, float downstream_pressure);
+
+    float indicated_afr() const { return indicated_afr_; }
+    float fuel_flow() const { return delivered_fuel_flow_; }
+    float film_mass() const { return film_mass_; }
+    float bowl_level() const { return bowl_mass_ / bowl_capacity; }
+    bool overrun_active() const { return overrun_; }
+    bool fault() const { return fault_; }
+    float venturi_depression() const { return venturi_depression_; }
+
+private:
+    float bowl_mass_{}, film_mass_{};
+    float throat_area_{}, venturi_depression_{};
+    float filtered_air_flow_{}, filtered_fuel_flow_{}, delivered_fuel_flow_{};
+    float indicated_afr_{13.5f};
+    float previous_pedal_{}, lift_enrichment_{};
+    bool initialized_{}, overrun_{}, fault_{};
+};
+
+} // namespace fluidsim::port