Add turbocharged twin scenario and cross-platform test harness

include/FluidSim/OutdoorAudio.hpp

@@ -0,0 +1,72 @@
+#pragma once
+
+#include "FluidSim/AudioProcessing.hpp"
+#include <array>
+#include <cstddef>
+#include <vector>
+
+namespace fluidsim::port {
+enum class OutdoorScene { Dry, Field, Wall, Woodland };
+const char *scene_name(OutdoorScene scene);
+
+struct OutdoorAudioConfig {
+    Vec3 listener{1.6f, 1.0f, 2.0f};
+    Vec3 listener_right{1.0f, 0.0f, 0.0f};
+    Vec3 intake_position{-0.2f, 0.7f, 0.0f};
+    Vec3 exhaust_position{0.55f, 0.5f, 0.0f};
+    float ear_spacing{0.18f}, sound_speed{343.0f};
+    float ground_y{0.0f}, wall_x{-1.5f};
+    float ground_reflection{0.35f}, wall_reflection{0.65f};
+    float pressure_gain{0.005f};      // 200 Pa per device full scale, not speaker SPL.
+    float monitor_cutoff_hz{6000.0f}; // Second-order acquisition/rendering filter.
+};
+
+// Static geometric direct/first-order reflection paths plus an explicitly
+// approximate diffuse tail. No feedback into the fluid/combustion simulation.
+class OutdoorAudio {
+public:
+    explicit OutdoorAudio(int sample_rate = 44100, OutdoorAudioConfig config = {});
+    AudioFrame process(float intake_mass_flow, float exhaust_mass_flow);
+    void set_scene(OutdoorScene scene) { scene_ = scene; }
+    OutdoorScene scene() const { return scene_; }
+    void set_monitor_filter(bool enabled) { monitor_filter_ = enabled; } // Set before playback/capture.
+    AudioFrame dry_frame() const { return last_dry_; }
+    AudioFrame unfiltered_frame() const { return last_raw_; }
+
+private:
+    struct LowPass {
+        float b0{}, b1{}, b2{}, a1{}, a2{}, z1{}, z2{};
+        float process(float x);
+    };
+    struct Path {
+        std::size_t delay_left{}, delay_right{};
+        float fraction_left{}, fraction_right{}, gain_left{}, gain_right{}, alpha{1};
+        float state_left{}, state_right{};
+        int group{}; // direct, ground, wall, trees
+    };
+    struct Emitter {
+        std::vector<float> filtered, raw;
+        std::vector<Path> paths;
+        std::size_t position{};
+        float previous_flow{};
+        LowPass filter;
+    };
+    struct TailLine {
+        std::vector<float> buffer;
+        std::size_t position{};
+        float gain{}, state{};
+    };
+    int rate_{};
+    OutdoorAudioConfig config_;
+    OutdoorScene scene_{OutdoorScene::Wall};
+    bool monitor_filter_{true};
+    std::array<Emitter, 2> emitters_;
+    std::array<TailLine, 4> tail_;
+    std::array<float, 4> weights_{1, 0, 0, 0};
+    float scene_alpha_{}, tail_alpha_{}, tail_mix_{};
+    AudioFrame last_dry_{}, last_raw_{};
+    void configure_emitter(Emitter &emitter, Vec3 position);
+    AudioFrame render(Emitter &emitter, float flow, AudioFrame &dry, AudioFrame &raw);
+    AudioFrame diffuse(float input);
+};
+} // namespace fluidsim::port