Add double-click bridge launchers; document ledtest + launchers in READMEs

start-bridge.bat (Windows) and start-bridge.command (macOS/Linux, +x) sit
next to bridge.py in both parts so readers can start the bridge with a
double-click. READMEs now cover the launchers and the ledtest.py debug path.

godot-arduino-1-led-first-light/arduino/bridge/start-bridge.command

@@ -0,0 +1,6 @@
+#!/usr/bin/env bash
+# Double-click in Finder (macOS) to start the Godot <-> Arduino serial bridge.
+# Keep this window open while you work; close it to stop the bridge.
+# On Linux this same file works once it's executable:  chmod +x start-bridge.command
+cd "$(dirname "$0")"
+python3 bridge.py