linux and windows workflows must never get disabled for inactivity again

[MisterGC]

GitHub switches scheduled workflows off after 60 days without commits; on 2026-09-21 three of the four tutorial/sample repos had them off. The KeepAlive workflow in zserio-tutorial-cpp only keeps itself alive. Also, a regeneration that produces a diff should go red instead of drifting.

Done when:

• every scheduled workflow has its own keep-alive step
• the regeneration step fails on a non-empty git diff