fix: drop the inert K prefix from the spending flow prefixes

K matches zero rows corpus-wide (audited pipeline-side). Numerically inert;
removed so the code stops implying a prefix the data never had.

tests/testthat/test-expenditure-concept.R

@@ -0,0 +1,14 @@
+test_that("the corpus contains no K-prefix rows, so the Direct leg omits K", {
+  con <- .ensure_session()
+  n <- DBI::dbGetQuery(con,
+    "SELECT COUNT(*) AS n FROM long WHERE LEFT(item_code, 1) = 'K'")$n
+  expect_equal(n, 0)
+
+  sql_files <- c("20-spending_long.sql", "22-spending_long_harmonized.sql")
+  for (f in sql_files) {
+    txt <- paste(readLines(system.file("sql", f, package = "uscogdata")),
+                 collapse = " ")
+    expect_false(grepl("'K'", txt, fixed = TRUE),
+                 label = paste(f, "must not reference the inert K prefix"))
+  }
+})