Nothing checks that lekkerzeilen still compiles

Found while measuring build time. The break that prompted this did not exist — see rejected/bug-n-0c508e507-... for my error. The gap is real regardless: nothing checks the demo builds. devdocs/perf/lekkerzeilen-build-time.md has timings a check could be budgeted against.

What makes this its own ticket: the false alarm will be forgotten by morning. The absence of any signal will not.