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/display_ui.h

@@ -0,0 +1,7 @@
+#pragma once
+#include <Arduino.h>
+#include "state.h"
+
+void displayBegin();
+void displaySplash(const char* line1, const char* line2);
+void displayRender(const AppState& s);