fix: let the doc-content tests survive R CMD check

CI failed on the previous commit. testthat::test_local() from a checkout was
green, but rcmdcheck was not: under R CMD check the suite runs against the
INSTALLED package, where README.md, vignettes/ and man/ do not exist. Both
newly-activated tests read them through test_path("..", "..", ...) and died
on `cannot open the connection`.

The defect was latent in the committed tests, not introduced here -- they
shipped skip()ped, so CI had never executed either one. Removing the skips
is what exposed it, which is the mechanism working as intended.

Guarded with skip_if_no_source_tree(), so they skip in the installed-package
context that structurally cannot satisfy them. They are NOT thereby unchecked
in CI: the workflow runs testthat::test_local() from the checkout as its own
step before rcmdcheck, and there the paths resolve and the assertions run.

Deliberately not split: test-peer-summary-scope.R's numeric pin needs only
the corpus and would survive check on its own, but it exists to protect the
sentence above it. Separating them would let the prose drift while the pin
kept passing.

Verified locally: test_local 629 pass / 0 fail / 3 skip; rcmdcheck
0 errors / 0 warnings / 0 notes.

tests/testthat/test-amount-units-documented.R

@@ -18,16 +18,23 @@
 
 test_that("returned amounts are documented as full US dollars where readers meet the package", {
 
+  # README and vignettes ship only in the source tree, not in the installed
+  # package, so these assertions cannot run under R CMD check -- CI's earlier
+  # testthat::test_local() step is what enforces them. See
+  # skip_if_no_source_tree() in helper-fixture.R.
+  docs <- skip_if_no_source_tree(
+    "README.md",
+    c("vignettes", "total-spending.Rmd"),
+    c("vignettes", "population-denominators.Rmd")
+  )
+
   says_units <- function(path) {
     txt <- paste(readLines(path, warn = FALSE), collapse = " ")
     grepl("full US dollars|full U\\.S\\. dollars", txt, ignore.case = TRUE) &&
       grepl("\\$1,000s|thousands of dollars", txt, ignore.case = TRUE)
   }
 
-  expect_true(says_units(testthat::test_path("..", "..", "README.md")))
-  expect_true(says_units(testthat::test_path("..", "..", "vignettes", "total-spending.Rmd")))
-  expect_true(says_units(testthat::test_path("..", "..", "vignettes",
-                                             "population-denominators.Rmd")))
+  for (path in docs) expect_true(says_units(path))
 
   # Pin the documented claim to the actual behaviour, so the two cannot drift.
   # The expected raw amount is read straight from the corpus's parquet