fix: literal name search, units docs, peer-summary semantics (#16, #15, #14)

The three kodor/fix issues, taken over after a day with no branch, PR or
comment on any of them. Batched because each is single-file with a committed
acceptance test, and two share documentation surfaces.

#16 (F-025) -- cog_gov_search() utility mode interpolated `name` straight
into regexp_matches() unescaped, while basket mode in the same file already
routed it through .escape_regex() with the comment "so `name` is treated as
a literal substring". Two failure modes, both HTTP 200 through the API:
a government could not be found by its own complete name when that name
contains a metacharacter (FREDONIA (BRISCOE) CITY returned nothing), and a
bare "." matched all 608 Wisconsin cities. Malformed pattern text reached
the engine as an error, which cog-api surfaced as a 500 -- reachable by
typing a real name one character at a time ("Athens-Clarke County (bal").

Utility mode now calls the escaper that already existed. Roxygen updated:
utility mode is documented as a literal case-insensitive substring match,
and the basket-mode "substring fallback" step no longer describes itself as
a regex either.

  BEHAVIOUR CHANGE worth flagging: anchored exact-match searches stop
  working, because there is no regex left to anchor. Two existing tests used
  "^BROWARD COUNTY$" and "^FLORIDA$" as their exact-match idiom; both now
  search for those characters literally. Updated to the bare names, which
  still resolve to exactly one row each once scoped by state/type (verified,
  not assumed). There is no exact-match option in utility mode any more --
  noted on the issue, since that is a real if small capability loss.

#15 (F-004) -- the raw Census files report thousands of dollars; this
package multiplies by 1000 and returns full US dollars. Correct, and already
stated in ?cog_spending / ?cog_revenue @return, in provenance, and in
cog-api's data-dictionary. Absent from every surface a reader meets FIRST.
Added to README.md as its own section and to both vignettes' openings.

The dangerous one is cog_explorer/CLAUDE.md, which states the opposite rule
("All raw `amt` values are in $1,000s") without scoping it to the raw column
-- a reader applying that to amt_nominal overstates by 1000x and gets a
plausible-looking number rather than an obvious error. Fixed there too; that
directory has no git remote, so it rides in no PR and is left uncommitted
for the owner.

#14 (F-021) -- .peer_summary_rows() computes stats::quantile() separately
inside each (year, spend_subtype, category) cell, so a summary_p50 row is
"the median peer's value in that one category", never "the value of the
median peer's total" -- the median peer for Police and for Fire are usually
different governments. Summing them across categories misstated a
total-spending band by -32.7% to +251.0% across 24 years, with a sign flip
at FY2012. The verb is right and its documented use (facet by role AND
category) is unaffected, so the fix is @return prose plus a worked snippet
showing the correct computation: sum each peer's own categories first, then
take the quantile of those per-government totals.

This is the R-side counterpart of cog-api#9, fixed on the API surface
earlier today; the wording is deliberately consistent across the two.

Note the phrase "not additive" has to stay on one roxygen source line --
the test greps the generated Rd, where a line wrap turns it into
"not   additive" and stops matching. Cost one red run to find.

man/ regenerated with roxygen 8.0.0 against a repo built with 7.3.3, so
cog_spending.Rd and DESCRIPTION were reverted -- their entire diff was
version churn (reindentation, RoxygenNote -> Config/roxygen2/version) with
no content change. The two Rd files kept carry only the edits above.

Suite: 629 pass / 0 fail / 3 skip (was 606/0/6). The three remaining skips
are #11, #12 and #13.

tests/testthat/test-rollup.R

@@ -80,8 +80,11 @@ test_that("cog_geographic_rollup provenance reports the outer verb", {
 
 test_that("cog_geographic_rollup accepts data.frames per layer", {
   skip_if_no_corpus()
-  fl_state  <- cog_gov_search("^FLORIDA$", type = "state")
-  broward   <- cog_gov_search("^BROWARD COUNTY$", state = "FL", type = "county")
+  # Unanchored: utility mode matches literally now, so "^...$" would be
+  # searched for as characters rather than read as anchors (uscogdata#16).
+  # Both still resolve to exactly one row once scoped by type/state.
+  fl_state  <- cog_gov_search("FLORIDA", type = "state")
+  broward   <- cog_gov_search("BROWARD COUNTY", state = "FL", type = "county")
   r <- cog_geographic_rollup(
     govids = list(state = fl_state, county = broward),
     category = "Police", years = 2020L