Word Guessing Game (Adivinar Pokemon)

Implement the logic for a Hangman-style word guessing game.

Write three pure functions:

1. getDisplay(word, guessed) — returns the word with unguessed letters as underscores (e.g. "P _ K A _ H U")
2. processGuess(letter, state) — processes a letter guess, returns new { guessed, wrong, gameOver, won }
3. isGameOver(state) — returns { won: boolean, lost: boolean }

The word is "PIKACHU", max 6 wrong guesses.