feat: add src/reading.cpp

src/reading.cpp

@@ -0,0 +1,10 @@
+#include "reading.hpp"
+#include <cmath>
+
+ChannelAssessment assess_channel(const Reading& reading, std::int64_t now_ms) {
+    if (reading.timestamp_ms > now_ms || now_ms - reading.timestamp_ms > 5000)
+        return {reading, ChannelState::Stale};
+    if (!std::isfinite(reading.celsius) || reading.celsius < -40.0 || reading.celsius > 125.0)
+        return {reading, ChannelState::OutOfRange};
+    return {reading, ChannelState::Healthy};
+}