Cursor test - #617

Add comprehensive test cases to test_tcp_socket.cpp that validate the
scoped receive-timeout guard feature described in issue #609. The guard
is an RAII helper on TCPSocket that temporarily raises the receive
timeout and restores it on destruction, replacing manual save/restore
patterns in the dashboard clients and the private helper in rtde_client.

Test cases cover:
- Basic raise and restore on scope exit
- Guard does not lower an already-higher timeout
- Guard raises timeout when current is lower
- Guard is a no-op when no timeout is configured (blocking socket)
- Timeout restored even when scope exits via exception
- Sub-second precision (e.g. 1.5s = 1s + 500000us)
- Nested guards restore in correct order
- Equal timeout request is a no-op
- Zero minimum is a no-op
- Move semantics (moved-from guard does not double-restore)