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/state.h

@@ -0,0 +1,15 @@
+#pragma once
+#include "rtc_time.h"
+#include "forecast.h"
+
+struct AppState {
+  float    absHpa;
+  float    mslHpa;
+  float    tempC;
+  Trend    trend;
+  Forecast forecast;
+  RtcTime  now;
+  bool     haveTrend;
+};
+
+extern AppState g_state;