Open Syzygy tablebase once at initialization

[luccabb]

Summary

• Open Syzygy tablebase once in __init__ instead of on every eval_board call
• Add close() method for proper resource cleanup
• Use pre-opened tablebase handle for all probes

Details

Previously, the tablebase was opened/closed on every call to eval_board() when
checking endgame positions:

# Before (slow)
with chess.syzygy.open_tablebase(path) as tablebase:
    tablebase.probe_dtz(board)

Now we open once and reuse:

# After (fast)
self.tablebase.probe_dtz(board)

This eliminates significant overhead in endgame positions where many evaluations
may hit the tablebase.

Test plan

• All 64 unit tests pass
• Tablebase functionality preserved

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