Add turbocharged twin scenario and cross-platform test harness

include/FluidSim/AudioOutputStream.hpp

@@ -12,6 +12,7 @@ class AudioOutputStream final : public sf::SoundStream {
 public:
     explicit AudioOutputStream(SimulationRingBuffer& ring, unsigned int sample_rate = 44100);
     void set_speed(double speed) { speed_.store(speed, std::memory_order_relaxed); }
+    std::uint64_t underrun_blocks() const { return underrun_blocks_.load(std::memory_order_relaxed); }
     double speed() const { return speed_.load(std::memory_order_relaxed); }
 
 private:
@@ -19,10 +20,16 @@ private:
     SimulationRingBuffer& ring_;
     std::array<float, block_size> temp_left_{}, temp_right_{};
     std::array<std::int16_t, block_size * 2> output_{};
+    std::atomic<std::uint64_t> underrun_blocks_{};
     std::atomic<double> speed_{1.0};
     float previous_input_left_{}, previous_output_left_{};
     float previous_input_right_{}, previous_output_right_{};
 
+    static constexpr std::size_t transition_samples = 64;
+    std::size_t transition_remaining_{};
+    float last_left_{}, last_right_{}, transition_left_{}, transition_right_{};
+    bool starving_{true};
+
     bool onGetData(Chunk& data) override;
     void onSeek(sf::Time time_offset) override;
 };