Fix audio driver fallback, NameError in InputBox, and add high score entry prompt

[Copilot]

Three bugs: audio fails after adding SDL driver fallback logic, NameError: name 'screen' is not defined crashes the game at game over when pressing Escape in the InputBox, and high score entry is never prompted regardless of score.

Audio driver env var restoration (src/ui/audio.py, missile-defense.py)

• SDL_AUDIODRIVER was restored to the original (possibly broken) value even after a working fallback driver was found — now only restores on failure
• missile-defense.py upgraded from bare try/except to the same multi-driver fallback loop (None → pulseaudio → alsa → dsp → dummy)

NameError in text.py

• exit_game(screen) referenced an undefined screen variable in module scope → replaced with pygame.display.get_surface()
• Removed pygame.display.update() from InputBox.draw() — caller now controls display updates, eliminating the stale white rectangle outline

High score entry flow (missile-defense.py, main.py)

• InputBox now only appears when check_high_score() confirms the player qualifies
• Entered initials are saved via update_high_scores() + save_high_scores() instead of hardcoded "---"
• main.py gains _prompt_initials() — a minimal input loop accepting up to 3 alphanumeric characters

# Before: always saved with placeholder name
self.high_scores = update_high_scores(score, "---", self.high_scores)

# After: only saves when qualified, prompts for name
score_pos = check_high_score(score, self.high_scores)
if score_pos > 0:
    name = self._prompt_initials()
    self.high_scores = update_high_scores(score, name, self.high_scores)
    save_high_scores(self.scores_file, self.high_scores)

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