test: bundle fixture corpus + wire local-path test helpers

Adds inst/extdata/fixture_corpus/ — a 3.6 MB two-year (2019/2020) slice
of the published corpus (OH+VT+WY fixture from cog_pipeline test profile
plus all 50 states). Includes canonical_fips_xwalk.parquet,
summary_categories.parquet, docs/, and a trimmed manifest.json.

setup.R now points USCOGDATA_URL at the bundled fixture automatically,
bypassing HTTP / Nextcloud entirely. DuckDB reads local parquet via the
existing .is_local_path() fast-path in manifest.R; no httpfs required.
session is reset between test files via withr::defer(cog_close()).

helper-fixture.R gains fixture_corpus_path(), a richer skip_if_no_corpus()
that checks the bundled fixture first, and with_fixture_corpus() for
tests that need explicit session isolation.

test-spending.R: adjust the inflate-column test to use 2019 (fixture year)
instead of 2015 (absent from fixture).

Result: 181 PASS / 0 FAIL / 0 SKIP — all tests run against real parquet
data with real DuckDB queries and no network dependency.

tests/testthat/helper-fixture.R

@@ -1,6 +1,28 @@
 # tests/testthat/helper-fixture.R
+
+# Returns the path to the bundled fixture corpus (trailing slash for DuckDB globs).
+fixture_corpus_path <- function() {
+  p <- system.file("extdata/fixture_corpus", package = "uscogdata")
+  if (nzchar(p)) paste0(p, "/") else ""
+}
+
+# Skip a test if no corpus is reachable (bundled fixture or explicit remote URL).
 skip_if_no_corpus <- function() {
-  testthat::skip_if(Sys.getenv("USCOGDATA_FIXTURE_URL", "") == "" &&
-                    !file.exists("~/.cache/R/uscogdata/manifest.json"),
-                    "No fixture corpus available")
+  p <- fixture_corpus_path()
+  has_fixture <- nzchar(p) && file.exists(sub("/$", "/manifest.json", p))
+  has_remote  <- nzchar(Sys.getenv("USCOGDATA_FIXTURE_URL", ""))
+  testthat::skip_if(!has_fixture && !has_remote, "No fixture corpus available")
+}
+
+# Run a block against the fixture corpus with a clean session.
+# Restores the previous URL and closes the DuckDB connection when done.
+with_fixture_corpus <- function(code) {
+  old_url <- Sys.getenv("USCOGDATA_URL", unset = NA)
+  uscogdata:::cog_close()
+  Sys.setenv(USCOGDATA_URL = fixture_corpus_path())
+  on.exit({
+    uscogdata:::cog_close()
+    if (is.na(old_url)) Sys.unsetenv("USCOGDATA_URL") else Sys.setenv(USCOGDATA_URL = old_url)
+  }, add = TRUE)
+  force(code)
 }