Add a tested C++ console input-validation experiment

[devkyato]

Answer the next question recorded in the Term 1 journal: how can console input errors be handled without leaving std::cin in a failed state?

Acceptance criteria

• add a small C++17 program that requests an integer in a documented range
• separate validation/calculation logic from console I/O where practical
• cover valid, out-of-range, and non-numeric input
• add automated tests without weakening -Werror / /W4
• add a journal entry describing the failure-state behavior and evidence

Keep this focused on input state; do not turn it into a large application.