Implement belief enumeration and fog_fen integration

[Belzedar94]

Summary

• add belief state enumeration for hidden opponent pieces and filter out inconsistent or illegal states
• parse fog_fen observations into belief state construction and seed the planner from partial FEN input
• update Obscuro and FoW guides to document the new enumeration and fog_fen integration

Testing

• not run (not requested)

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Codex Task