fix: push limit/offset into the query instead of materializing then slicing

cog-api's paginate() sliced an already-fully-materialized result: every
page of a deep sweep re-ran the whole cog_spending()/cog_revenue() query
and re-listified every row, just to keep up to 1000 and discard the
rest. A 193,105-row/194-page fleet-wide sweep (cog_explorer's Southern
guide, corpus summary build) repeated that full cost 194 times and
wedged the production server for hours on 2026-08-06 -- single request,
CPU-bound, single-threaded plumber process, no other request could get
through, not even /health.

cog_spending()/cog_revenue() gain optional limit/offset, pushed into
.build_verb_sql() as SQL LIMIT/OFFSET behind the existing (already
deterministic) ORDER BY. The full unpaginated row count rides along via
COUNT(*) OVER() in the same scan -- exposed as a total_rows attribute --
so a caller walking pages never needs a second round trip to ask how
many there are. A page now costs O(limit), not O(full result).

Mutually exclusive with complete = TRUE (which fills a grid over the
FULL requested (year, category) space -- pagination over a partial slice
of already-grouped rows has no defined meaning for the cells it would
fill) and with recipe (whose result comes from a separate, not-yet-wired
query path). Both abort with a clear classed condition rather than
silently ignoring the parameter.

limit/offset default to NULL; every existing call site is unaffected.

tests/testthat/test-spending-pagination.R

@@ -0,0 +1,95 @@
+# cog-api's paginate() used to slice an already-fully-materialized result:
+# every page of a deep sweep re-ran the whole query and re-listified every
+# row, just to keep 1000 and discard the rest. For a 193,105-row fleet-wide
+# query walked 194 pages deep, that repeated the full cost 194 times and
+# wedged the production server for hours (2026-08-06 incident). limit/offset
+# here push the slice into the SQL itself, so a page costs O(limit), not
+# O(full result).
+
+test_that("limit without offset returns the first page, matching the unpaginated head", {
+  skip_if_no_corpus()
+  full <- cog_spending("121011212191", years = 2019:2020, category = NULL)
+  page <- cog_spending("121011212191", years = 2019:2020, category = NULL,
+                       limit = 10L)
+  expect_equal(nrow(page), 10L)
+  expect_equal(page[c("year", "canonical_govid", "spend_subtype", "category")],
+              full[1:10, c("year", "canonical_govid", "spend_subtype", "category")],
+              ignore_attr = TRUE)
+})
+
+test_that("offset skips ahead without gaps or overlap", {
+  skip_if_no_corpus()
+  full <- cog_spending("121011212191", years = 2019:2020, category = NULL)
+  page2 <- cog_spending("121011212191", years = 2019:2020, category = NULL,
+                        limit = 10L, offset = 10L)
+  expect_equal(nrow(page2), 10L)
+  expect_equal(page2[c("year", "canonical_govid", "spend_subtype", "category")],
+              full[11:20, c("year", "canonical_govid", "spend_subtype", "category")],
+              ignore_attr = TRUE)
+})
+
+test_that("walking every page reconstructs the unpaginated result exactly", {
+  skip_if_no_corpus()
+  full <- cog_spending("121011212191", years = 2019:2020, category = NULL)
+  n <- nrow(full)
+  limit <- 7L
+  pages <- list()
+  offset <- 0L
+  repeat {
+    p <- cog_spending("121011212191", years = 2019:2020, category = NULL,
+                      limit = limit, offset = offset)
+    if (nrow(p) == 0L) break
+    pages[[length(pages) + 1L]] <- p
+    offset <- offset + limit
+    if (offset > n + limit) stop("test runaway: paging did not terminate")
+  }
+  walked <- dplyr::bind_rows(pages)
+  expect_equal(nrow(walked), n)
+  key_cols <- c("year", "canonical_govid", "spend_subtype", "category", "amt_nominal")
+  expect_equal(walked[key_cols], full[key_cols], ignore_attr = TRUE)
+})
+
+test_that("total_rows attribute reports the full unpaginated count", {
+  skip_if_no_corpus()
+  full <- cog_spending("121011212191", years = 2019:2020, category = NULL)
+  page <- cog_spending("121011212191", years = 2019:2020, category = NULL,
+                       limit = 5L, offset = 0L)
+  expect_equal(attr(page, "total_rows"), nrow(full))
+})
+
+test_that("offset past the end returns zero rows, not an error", {
+  skip_if_no_corpus()
+  full <- cog_spending("121011212191", years = 2019:2020, category = NULL)
+  page <- cog_spending("121011212191", years = 2019:2020, category = NULL,
+                       limit = 10L, offset = nrow(full) + 100L)
+  expect_equal(nrow(page), 0L)
+  expect_equal(attr(page, "total_rows"), nrow(full))
+})
+
+test_that("limit is unset by default -- unpaginated calls are unaffected", {
+  skip_if_no_corpus()
+  r <- cog_spending("121011212191", 2020L, "Corrections")
+  expect_null(attr(r, "total_rows"))
+})
+
+test_that("per_capita and adjust_to_year still apply correctly within a page", {
+  skip_if_no_corpus()
+  full <- cog_spending("121011212191", years = 2020L, category = NULL,
+                       per_capita = TRUE, adjust_to_year = 2022L)
+  page <- cog_spending("121011212191", years = 2020L, category = NULL,
+                       per_capita = TRUE, adjust_to_year = 2022L,
+                       limit = 3L, offset = 2L)
+  expect_equal(page[c("amt_nominal", "amt_real", "amt_per_capita_nominal",
+                      "amt_per_capita_real")],
+              full[3:5, c("amt_nominal", "amt_real", "amt_per_capita_nominal",
+                          "amt_per_capita_real")],
+              ignore_attr = TRUE)
+})
+
+test_that("complete = TRUE with limit aborts -- pagination over a partial grid is undefined", {
+  skip_if_no_corpus()
+  expect_error(
+    cog_spending("121011212191", 2020L, "Corrections", complete = TRUE, limit = 5L),
+    class = "uscogdata_complete_pagination_conflict"
+  )
+})