feat: bundle CPIAUCSL + .inflate() helper

Adds R/sysdata.rda with the annual-average CPIAUCSL index (1947-2026,
80 years, sourced from FRED) and an internal .inflate() helper that
converts nominal amounts between two years via the ratio of CPI values.

Bundling CPI in the package (rather than publishing a cpi_annual.parquet
in the corpus) matches the reader-specification intent: real-dollar
conversion is a verb-level option, not a corpus-level artifact, so the
target_year stays flexible at query time.

data-raw/cpi_annual.R carries the one-shot FRED refresh used to build
sysdata.rda. Re-run when the CPI series needs to roll forward.

Verified: CPI(2000)/CPI(2021) ≈ 0.635, matching the ~0.63 sanity
anchor in cog_explorer's existing inflation logic.

Tests: tests/testthat/test-adjust.R covers the known 2000→2021 ≈ 1.574x
anchor, vectorized from_year, error cases for out-of-range years, and
NA-amount preservation. 14 pass / 0 fail.

tests/testthat/test-adjust.R

@@ -0,0 +1,43 @@
+test_that("cpi_annual internal data is available with expected shape", {
+  cpi <- uscogdata:::.cpi_table()
+  expect_true(is.data.frame(cpi))
+  expect_named(cpi, c("year", "cpi"))
+  expect_true(nrow(cpi) > 70L)
+  expect_true(all(c(2000L, 2010L, 2021L) %in% cpi$year))
+})
+
+test_that(".inflate matches a known anchor: 2000 in 2021$ ≈ 1.57x nominal", {
+  # FRED CPIAUCSL annual: 2000=172.19, 2021=270.97 → ratio ≈ 1.574
+  result <- uscogdata:::.inflate(100, from_year = 2000, to_year = 2021)
+  expect_equal(result, 157.4, tolerance = 0.5)
+})
+
+test_that(".inflate is vectorized over from_year", {
+  result <- uscogdata:::.inflate(
+    amt       = c(100, 100, 100),
+    from_year = c(2000, 2010, 2021),
+    to_year   = 2021
+  )
+  expect_length(result, 3L)
+  expect_equal(result[3], 100, tolerance = 1e-6)  # same year → identity
+  expect_gt(result[1], 150)  # 2000 inflated to 2021 ≈ 157
+  expect_lt(result[2], 130)  # 2010 inflated to 2021 ≈ 124
+  expect_gt(result[2], 115)
+})
+
+test_that(".inflate errors on unknown from_year or to_year", {
+  expect_error(
+    uscogdata:::.inflate(100, from_year = 1900, to_year = 2021),
+    "CPI unavailable for from_year"
+  )
+  expect_error(
+    uscogdata:::.inflate(100, from_year = 2000, to_year = 2200),
+    "CPI unavailable for to_year"
+  )
+})
+
+test_that(".inflate preserves NA amounts", {
+  result <- uscogdata:::.inflate(c(100, NA, 200), from_year = 2000, to_year = 2021)
+  expect_true(is.na(result[2]))
+  expect_false(any(is.na(result[c(1, 3)])))
+})