Manage XBoard ponder thread lifecycle

[RainRat]

Summary

• keep the ponder worker started by the XBoard state machine joinable and under explicit management
• add helpers on the state machine to stop or join a running ponder worker before reuse or destruction
• call the new shutdown helper when stopping the engine and during program exit to avoid dangling threads

Testing

• not run (not provided)

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https://chatgpt.com/codex/tasks/task_e_68cf1ed7c0288330a4a163b35c53817d