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$

tests/testthat/test-config.R

@@ -122,3 +122,12 @@ test_that("LICENSE and LICENSE.md name the same copyright holder", {
   # LICENSE alone, which is how the two came to disagree in the first place.
   expect_match(full, "MIT License", fixed = TRUE)
 })
+
+test_that("vignettes are not excluded from the build", {
+  skip_if_no_source_tree(".Rbuildignore")
+  ignore <- readLines(source_tree_path(".Rbuildignore"), warn = FALSE)
+  expect_false(any(grepl("^\\^vignettes\\$$", ignore)))
+  # The fixture is what lets R CMD check run offline with no credentials on
+  # r-universe and GitHub Actions. It must never be excluded.
+  expect_false(any(grepl("fixture_corpus", ignore, fixed = TRUE)))
+})