polish(per-year-pop): expand popyear in cog_explain + propagate pop_range

Final-review followups:

1. cog_explain rendered the popyear range as raw 2-digit values
   ("popyear range: 19-20"), which a user could read as the years 19-20.
   Added .expand_popyear() helper to format as 4-digit calendar years
   (2019-2020). Pivot at 70 to handle pre-2000 vintages if the corpus
   ever extends backward.

2. cog_peer_compare provenance was missing pop_range and is_ratio,
   omitted from the spec-required reproducibility metadata.
   cog_find_peers now stamps both as tibble attributes; cog_peer_compare
   reads them through to provenance$pop_range and provenance$is_ratio.

Test coverage extended: explain test asserts the 4-digit format and
rejects the old 2-digit form; peer-compare test asserts pop_range +
is_ratio propagate end-to-end.

326 PASS / 0 FAIL.

tests/testthat/test-explain.R

@@ -43,5 +43,8 @@ test_that("cog_explain prints denominator + popyear_range + counts", {
     expect_true(grepl("Census F-33", out))
     expect_true(grepl("popyear", out, ignore.case = TRUE))
     expect_true(grepl("census_f33", out))
+    # popyear_range should render as 4-digit calendar years, not raw 2-digit
+    expect_true(grepl("2019-2020", out))
+    expect_false(grepl("popyear range: 19-20", out, fixed = TRUE))
   })
 })