Main game loop: fortification phase

[benrs]

Provide a group of C++ classes that implement the fortification phase following the official rules of the game of
Risk. In the fortification phase, the player is allowed to move a number of armies (X) from one of its countries (the
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source country) to one of its neighbors that it also owns (the target country). X must be in the range [1 to (number
of armies on the source country - 1)]. You must deliver a driver that demonstrates that 1) only valid countries can
be selected as source/target; 2) only a valid number of armies can be moved; 3) the right number of armies is
effectively moved.