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

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

WeatherPredictor/WeatherPredictor.ino

@@ -5,6 +5,7 @@
 #include "sensors.h"
 #include "rtc_time.h"
 #include "forecast.h"
+#include "history.h"
 
 static void i2cScan() {
   Serial.println(F("I2C scan:"));
@@ -46,6 +47,18 @@ void setup() {
     Serial.printf("p=%.0f trend=%d -> %c [%s] (%s)\n",
                   c.p, c.tr, f.letter, f.text, categoryShort(f.category));
   }
+
+  // --- history self-test (remove after Task 8) ---
+  historyBegin();
+  uint32_t base = 1000000000UL;
+  for (int i = 0; i < 40; i++)                 // 40 samples @ 5 min = 3h20m, pressure falling
+    historyAdd(base + (uint32_t)i * 300, 1015.0f - i * 0.2f, 20.0f);
+  float d;
+  if (historyTrendDelta(d))
+    Serial.printf("history count=%d  3h delta=%.2f hPa  trend=%d\n",
+                  historyCount(), d, classifyTrend(d));
+  else
+    Serial.println(F("history: not enough data for trend"));
 }
 
 void loop() {