Replace boost::signals2 with std::function

[Copilot]

Removes boost dependency by replacing boost::signals2::signal with std::function for UCI protocol callbacks.

Changes

• Header (uci.hpp): Replaced all 11 signal declarations with std::function, added null checks before invocation
• CMakeLists.txt: Removed boost find_package requirement
• API: Callbacks now use direct assignment instead of .connect() method
• Documentation: Updated usage examples and documented single-subscriber limitation

API Change

Before:

uci.receive_uci.connect([&]() {
  uci.send_id("Engine", "Author");
  uci.send_uci_ok();
});

After:

uci.receive_uci = [&]() {
  uci.send_id("Engine", "Author");
  uci.send_uci_ok();
};

Limitations

• Each callback now supports single subscriber only (boost::signals2 supported multicast)
• No built-in thread safety (boost::signals2 provided this)

Original prompt

Replace the boost signals dependency with std::function.

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