feat: Zambretti forecast with trend classification

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

WeatherPredictor/WeatherPredictor.ino

@@ -4,6 +4,7 @@
 #include "display_ui.h"
 #include "sensors.h"
 #include "rtc_time.h"
+#include "forecast.h"
 
 static void i2cScan() {
   Serial.println(F("I2C scan:"));
@@ -34,6 +35,17 @@ void setup() {
     Serial.println(F("RTC lost power -> setting to build time"));
     rtcSet(RtcTime{2026, 7, 17, 12, 0, 0});  // temporary; NTP will correct later
   }
+
+  // --- forecast self-test (remove after Task 8) ---
+  struct { float p; Trend tr; } cases[] = {
+    {1030, TREND_STEADY}, {1030, TREND_RISING}, {1030, TREND_FALLING},
+    {1000, TREND_STEADY}, {1000, TREND_FALLING}, {970, TREND_FALLING},
+  };
+  for (auto& c : cases) {
+    Forecast f = computeForecast(c.p, c.tr, 7);
+    Serial.printf("p=%.0f trend=%d -> %c [%s] (%s)\n",
+                  c.p, c.tr, f.letter, f.text, categoryShort(f.category));
+  }
 }
 
 void loop() {