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.

R/adjust.R

@@ -0,0 +1,45 @@
+# R/adjust.R
+# Inflation adjustment helpers using bundled CPIAUCSL annual averages.
+# The `cpi_annual` tibble (year, cpi) is stored as internal data in
+# R/sysdata.rda and built via data-raw/ on package update.
+
+#' Return the bundled annual CPI table.
+#'
+#' @return Tibble with columns `year` (integer) and `cpi` (numeric, CPIAUCSL
+#'   annual average, 1982-84 = 100).
+#' @noRd
+.cpi_table <- function() {
+  cpi_annual
+}
+
+#' Inflate (or deflate) an amount vector between two years.
+#'
+#' Converts nominal amounts in `from_year` dollars to real amounts in
+#' `to_year` dollars using the bundled CPIAUCSL annual average index.
+#' Multiplies by `cpi[to_year] / cpi[from_year]`.
+#'
+#' @param amt Numeric vector of nominal amounts.
+#' @param from_year Integer or integer-like vector of source years (one per
+#'   element of `amt`, or length 1).
+#' @param to_year Integer target year (scalar).
+#' @return Numeric vector of real amounts, same length as `amt`.
+#' @noRd
+.inflate <- function(amt, from_year, to_year) {
+  cpi <- .cpi_table()
+  from_year <- as.integer(from_year)
+  to_year   <- as.integer(to_year)
+  if (length(to_year) != 1L) {
+    cli::cli_abort("`to_year` must be a scalar.")
+  }
+  if (!(to_year %in% cpi$year)) {
+    cli::cli_abort("CPI unavailable for to_year = {to_year}. Supported: {min(cpi$year)}-{max(cpi$year)}.")
+  }
+  missing_years <- setdiff(from_year[!is.na(from_year)], cpi$year)
+  if (length(missing_years) > 0) {
+    cli::cli_abort("CPI unavailable for from_year value(s): {missing_years}.")
+  }
+  lookup <- stats::setNames(cpi$cpi, as.character(cpi$year))
+  cpi_from <- unname(lookup[as.character(from_year)])
+  cpi_to   <- unname(lookup[as.character(to_year)])
+  amt * cpi_to / cpi_from
+}