Keep engine process alive between moves

[Copilot]

The engine spawns a new process and quits after every single move. This change persists the engine process across moves within a game, quitting only on game over or mode exit (Neutral, New Game, Resign, etc.).

Changes

• RunEngine.__init__: Accept optional existing_engine parameter to reuse a running engine instance
• RunEngine.run(): Skip popen_uci spawn and configure_engine when an existing engine is provided
• RunEngine.get_engine(): New method — returns the engine instance without quitting, enabling hand-off between moves
• RunEngine.quit_engine(): Guard against None, set self.engine = None after quit
• play_game(): Track persistent_engine across the game loop; retrieve via get_engine() after each move, quit once on loop exit

Usage pattern

# Before: engine quit after every move
search.join()
search.quit_engine()

# After: engine handed off for reuse, quit only at end of game
search.join()
persistent_engine = search.get_engine()

# On next engine turn, reuse it
search = RunEngine(..., existing_engine=persistent_engine)

# Quit only when game loop exits (game over, neutral, resign, etc.)
if persistent_engine is not None:
    persistent_engine.quit()