feat: add include/fusion.hpp

include/fusion.hpp

@@ -0,0 +1,8 @@
+#pragma once
+#include "reading.hpp"
+#include <optional>
+
+enum class FusionState { Nominal, Degraded, Conflict, Unavailable };
+struct FusionResult { FusionState state; std::optional<double> celsius; std::string source; };
+FusionResult fuse(const ChannelAssessment& primary, const ChannelAssessment& secondary);
+const char* state_name(FusionState state);