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.

.Rbuildignore

@@ -1,3 +1,4 @@
+^data-raw$
 ^.*\.Rproj$
 ^\.Rproj\.user$
 ^_pkgdown\.yml$