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.

data-raw/cpi_annual.R

@@ -0,0 +1,34 @@
+# data-raw/cpi_annual.R
+#
+# Refresh the bundled CPIAUCSL annual-average table from FRED.
+# Run interactively when the CPI series needs to be extended forward:
+#   Rscript data-raw/cpi_annual.R
+#
+# Source: FRED series CPIAUCSL (Consumer Price Index for All Urban Consumers,
+# All Items, 1982-84 = 100), monthly. Annual mean computed here. The bundled
+# artifact is R/sysdata.rda (loaded automatically by the package).
+
+stopifnot(requireNamespace("utils", quietly = TRUE),
+          requireNamespace("tibble", quietly = TRUE),
+          requireNamespace("usethis", quietly = TRUE))
+
+fred_url <- "https://fred.stlouisfed.org/graph/fredgraph.csv?id=CPIAUCSL"
+raw <- utils::read.csv(url(fred_url), stringsAsFactors = FALSE)
+date_col <- intersect(c("observation_date", "DATE", "date"), names(raw))[1]
+stopifnot(length(date_col) == 1L, !is.na(date_col))
+raw$year <- as.integer(format(as.Date(raw[[date_col]]), "%Y"))
+raw$CPIAUCSL <- suppressWarnings(as.numeric(raw$CPIAUCSL))
+raw <- raw[!is.na(raw$CPIAUCSL), ]
+
+annual <- stats::aggregate(
+  raw$CPIAUCSL, by = list(year = raw$year), FUN = mean, na.rm = TRUE
+)
+names(annual)[2] <- "cpi"
+
+cpi_annual <- tibble::as_tibble(annual)
+cpi_annual$cpi <- round(cpi_annual$cpi, 4)
+
+message(sprintf("CPI range: %d-%d (%d years)",
+                min(cpi_annual$year), max(cpi_annual$year), nrow(cpi_annual)))
+
+usethis::use_data(cpi_annual, internal = TRUE, overwrite = TRUE)