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/setup.R

@@ -1,5 +1,13 @@
 # tests/testthat/setup.R
-# Point tests at a fixture corpus URL if provided.
-if (Sys.getenv("USCOGDATA_FIXTURE_URL", "") != "") {
-  options(uscogdata.url = Sys.getenv("USCOGDATA_FIXTURE_URL"))
-}
+# Priority: bundled fixture > USCOGDATA_FIXTURE_URL env var > whatever is already set.
+local({
+  p <- system.file("extdata/fixture_corpus", package = "uscogdata")
+  if (nzchar(p) && file.exists(file.path(p, "manifest.json"))) {
+    Sys.setenv(USCOGDATA_URL = paste0(p, "/"))
+  } else if (nzchar(Sys.getenv("USCOGDATA_FIXTURE_URL", ""))) {
+    Sys.setenv(USCOGDATA_URL = Sys.getenv("USCOGDATA_FIXTURE_URL"))
+  }
+})
+
+# Reset DuckDB session between test files so each file starts clean.
+withr::defer(uscogdata:::cog_close(), teardown_env())