.Rbuildignore excluded ^vignettes$, ^doc$ and ^Meta$, so an installed
uscogdata had no vignettes at all -- while the README instructed users to
run vignette("total-spending"), which failed for every one of them.
Both build offline: total-spending points USCOGDATA_URL at the bundled
fixture, population-denominators is eval = FALSE. Confirmed present in the
built tarball as both source and rendered inst/doc/.
The test also pins the fixture as never-excluded -- it is what lets
R CMD check run with no credentials on r-universe and GitHub Actions.