Add Arduino answer key (4 hardware activities, teacher reference)

[beagandica]

Summary

Add a teacher reference answer key for the Arduino Light Game workshop (Elegoo UNO starter kit).

Changes

• content/english/arduino/answer-key.md (new, 310 lines)
  • Activity 1: Blinking LED (built-in example, timing challenge answer)
  • Activity 2: Joystick with serial output (full code, expected output)
  • Activity 3: Joystick + LEDs (fixed code with 3 workshop bugs flagged)
  • Activity 4: LCD Hello World (full code, pin mapping table)
  • Appendix: Component reference table

Workshop source bugs flagged for teachers

• Activity 3: joyX and joyY used but never declared (code won't compile)
• Activity 3: VRy wired to GND instead of A1 (Y-axis won't read)
• Activity 3: Pin 7 LED in code but not in wiring steps
• Activity 4: Text says 5K resistor, code comment says 10K (either works)
• Activity 3: Serial output says "Switch = High" when buttonState is LOW

Languages affected

• English only. PT-BR translation exists but no answer key (tracked in backlog).

Testing

• Hugo build passes
• All Arduino code verified for compilation
• 10-model QA passed (3 clean convergence passes)