[BUG] TicTacToe `check_fork_opportunities()` corrupts board state

[galpt]

Bug Description

In cog/games.py#L302-L320, the check_fork_opportunities() function corrupts the board state when it finds a fork opportunity.

def check_fork_opportunities(self):
    fork_candidates = []
    for idx in empty:
        self.board[idx] = bot_mark  # <-- corrupts board
        win_count = 0
        for i in range(self.size * self.size):
            if self.board[i] == bot_mark:
                continue
            self.board[i] = bot_mark  # <-- corrupts board
            if self.check_winner(bot_mark):
                win_count += 1
            self.board[i] = " "  # Resets but...
        if win_count >= 2:
            fork_candidates.append((win_count, idx))
        self.board[idx] = " "  # Only resets idx, not line 310 corruption
    if fork_candidates:
        return max(fork_candidates)[1]  # Returns early without full cleanup!
    return None

Impact: When the function returns early (when fork_candidates is non-empty), the board is left with extra bot marks. Subsequent strategy checks (check_block_fork, get_corner_strategy, etc.) operate on corrupted board state, leading to incorrect bot decisions.

---

How to Fix

Track ALL modified cells and restore them:

def check_fork_opportunities(self):
    fork_candidates = []
    for idx in empty:
        self.board[idx] = bot_mark
        win_count = 0
        modified = [idx]  # Track all modified cells
        for i in range(self.size * self.size):
            if self.board[i] == bot_mark:
                continue
            self.board[i] = bot_mark
            modified.append(i)
            if self.check_winner(bot_mark):
                win_count += 1
            self.board[i] = " "
        if win_count >= 2:
            fork_candidates.append((win_count, idx))
        for m in modified:  # Restore ALL modified cells
            self.board[m] = " "
    if fork_candidates:
        return max(fork_candidates)[1]
    return None

---

Severity

Critical - Causes incorrect bot AI decisions after fork detection

[galpt]

[STILL PRESENT] check_fork_opportunities() has board state restoration pattern that could leave corruption if exception occurs. The function mutates board during simulation without try/finally guarantee.