Added

• dev check: Check that all exercises (including third-party ones) include at least one TODO comment.
• dev check: Check that all exercises actually fail to run (not already solved).

Changed

• Make enum variants more consistent between enum exercises.
• iterators3: Teach about the possible case of integer overflow during division.

Fixed

• Exit with a helpful error message on missing/unsupported terminal/TTY.
• Mark the last exercise as done.