chore: make repo an Arduino collection with WeatherPredictor project

Flatten so the repo root is the Arduino sketchbook; the project lives in
WeatherPredictor/. Add a collection index README at the root and move the
detailed project README into WeatherPredictor/.

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

WeatherPredictor/forecast.h

@@ -0,0 +1,16 @@
+#pragma once
+#include <Arduino.h>
+
+enum Trend { TREND_FALLING = -1, TREND_STEADY = 0, TREND_RISING = 1 };
+enum WxCategory { WX_FINE, WX_FAIR, WX_CHANGEABLE, WX_RAIN, WX_STORM, WX_UNKNOWN };
+
+struct Forecast {
+  char        letter;    // 'A'..'Z'
+  const char* text;      // full Zambretti phrase
+  WxCategory  category;  // for display icon + short label
+};
+
+Trend       classifyTrend(float deltaHpa3h);
+Forecast    computeForecast(float mslHpa, Trend trend, int month);
+const char* categoryShort(WxCategory c);
+const char* forecastTextForLetter(char letter);   // full Zambretti phrase for 'A'..'Z'