Add turbocharged twin scenario and cross-platform test harness

include/FluidSim/PipeProfile.hpp

@@ -0,0 +1,34 @@
+#pragma once
+#include "FluidSim/Fluid.hpp"
+#include <span>
+
+namespace fluidsim::port {
+struct PipeSection {
+    float length, inlet_diameter, outlet_diameter;
+};
+
+// Linear diameter within each section. Cell area is the exact frustum-volume
+// average, so refining the mesh preserves the specified chamber volume.
+inline Pipe1D make_profiled_pipe(std::span<const PipeSection> sections, float maximum_cell_length = .01f) {
+    if (!std::isfinite(maximum_cell_length) || maximum_cell_length <= 0 || sections.empty())
+        throw std::invalid_argument("invalid pipe profile mesh");
+    std::vector<float> areas, lengths;
+    float previous_diameter = sections.front().inlet_diameter;
+    for (const auto& section : sections) {
+        if (!std::isfinite(section.length) || !std::isfinite(section.inlet_diameter) ||
+            !std::isfinite(section.outlet_diameter) || section.length <= 0 ||
+            section.inlet_diameter <= 0 || section.outlet_diameter <= 0 ||
+            std::abs(section.inlet_diameter - previous_diameter) > 1e-6f)
+            throw std::invalid_argument("invalid/discontinuous pipe profile");
+        const int count = static_cast<int>(std::ceil(section.length / maximum_cell_length));
+        for (int i = 0; i < count; ++i) {
+            const float d0 = std::lerp(section.inlet_diameter, section.outlet_diameter, float(i) / count);
+            const float d1 = std::lerp(section.inlet_diameter, section.outlet_diameter, float(i + 1) / count);
+            areas.push_back(pi / 12 * (d0*d0 + d0*d1 + d1*d1));
+            lengths.push_back(section.length / count);
+        }
+        previous_diameter = section.outlet_diameter;
+    }
+    return Pipe1D(std::move(areas), std::move(lengths));
+}
+} // namespace fluidsim::port