fix: ship the vignettes

.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.

.Rbuildignore

@@ -9,11 +9,8 @@
 ^\.git$
 ^\.gitignore$
 \.gitkeep$
-^vignettes$
 ^specs$
 ^plans$
-^doc$
-^Meta$
 ^\.gitea$
 ^CLAUDE\.md$
 ^\.superpowers$