The bug was caused by accessing event.key.code on a mouse event without
checking the event type first. Since SFML uses a union for events, this
read garbage data. The middle mouse button value (2) coincidentally matched
the keyboard 'C' value (2), causing the spurious keyboard event.
Fixed by adding event type check before accessing key-specific fields.
Only keyboard events (KeyPressed/KeyReleased) now trigger key callbacks.
Test added to verify middle clicks no longer generate keyboard events.
Closes#78
