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

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

WeatherPredictor/history.cpp

@@ -0,0 +1,38 @@
+#include "config.h"
+#include "history.h"
+
+static Sample s_buf[HISTORY_SIZE];
+static int    s_head  = 0;   // next write position
+static int    s_count = 0;
+
+void historyBegin() { s_head = 0; s_count = 0; }
+
+void historyAdd(uint32_t epoch, float mslHpa, float tempC) {
+  s_buf[s_head] = Sample{ epoch, mslHpa, tempC };
+  s_head = (s_head + 1) % HISTORY_SIZE;
+  if (s_count < HISTORY_SIZE) s_count++;
+}
+
+int historyCount() { return s_count; }
+
+Sample historyGet(int i) {
+  int start = (s_head - s_count + HISTORY_SIZE) % HISTORY_SIZE;
+  return s_buf[(start + i) % HISTORY_SIZE];
+}
+
+Sample historyLatest() { return historyGet(s_count - 1); }
+
+bool historyTrendDelta(float& outDelta) {
+  if (s_count < 2) return false;
+  Sample latest = historyLatest();
+  uint32_t target = latest.epoch - 10800UL;   // 3 h earlier
+  int idx = -1;
+  for (int i = 0; i < s_count; i++) {
+    if (historyGet(i).epoch <= target) idx = i; else break;
+  }
+  if (idx < 0) return false;
+  Sample past = historyGet(idx);
+  if (latest.epoch - past.epoch < 9000UL) return false;  // need >= 2.5 h span
+  outDelta = latest.mslHpa - past.mslHpa;
+  return true;
+}