feat: in-RAM pressure history ring buffer with 3h trend

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

WeatherPredictor/history.h

@@ -0,0 +1,15 @@
+#pragma once
+#include <Arduino.h>
+
+struct Sample {
+  uint32_t epoch;
+  float    mslHpa;
+  float    tempC;
+};
+
+void   historyBegin();
+void   historyAdd(uint32_t epoch, float mslHpa, float tempC);
+int    historyCount();
+Sample historyGet(int i);      // 0 = oldest
+Sample historyLatest();
+bool   historyTrendDelta(float& outDeltaHpa);