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-2-joystick-dashboard/arduino/bridge/start-bridge.bat

@@ -0,0 +1,8 @@
+@echo off
+REM Double-click to start the Godot <-> Arduino serial bridge (Windows).
+REM Keep this window open while you work; close it to stop the bridge.
+cd /d "%~dp0"
+py bridge.py
+echo.
+echo Bridge stopped. Press any key to close this window.
+pause >nul