Fixtures whose every need is built but whose types or expressions the rig catalog (the reference .dat files,
no server) cannot answer: user domains / types / functions created by the test, xml, and one syscache seam the rig
never installs. The server runner judges them. One file per fixture, `<fixture>.tsv`, one line: why. A new entry
needs the same cause.

One file per entry (Michael 09-24): lanes never edit one line; scripts/gravity-gate/m2-decompose.py.
