make key-fold-check is undocumented and nothing checks that every Makefile target is described #1275

Where: CONTRIBUTING.md, Makefile.

Current gap: Every documented target is listed in CONTRIBUTING except help and key-fold-check, so the check that the JS case-folding table still matches Python is not mentioned where contributors look.

Suggested approach: Document make key-fold-check and add a test that each target with a ## description appears as make <target> in CONTRIBUTING (with help as the one exception).

Done when: the test fails when a new target is added without a CONTRIBUTING line.