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.

man/cog_gov_search.Rd

@@ -32,8 +32,11 @@ the cross-vintage canonical-government registry. Operates in two modes:
 }
 \details{
 * **Utility mode** (single `name`, the original behavior): returns all
-  rows whose `gov_name` matches the regex case-insensitively, sorted by
-  `population_acs` descending. Useful for exploratory lookups.
+  rows whose `gov_name` contains `name` as a **literal, case-insensitive
+  substring**, sorted by `population_acs` descending. Useful for
+  exploratory lookups. Regex metacharacters in `name` are escaped, so a
+  government is findable by its own complete name even when that name
+  contains parentheses or a period.
 * **Basket mode** (`length(name) > 1`): resolves each input row to a
   single canonical govid and returns a tibble in input order, suitable
   for piping straight into [cog_spending()] / [cog_revenue()] /
@@ -45,7 +48,8 @@ the cross-vintage canonical-government registry. Operates in two modes:
 1. Filter `canonical_fips_xwalk` by `state` and (if non-NA) `type`.
 2. **Exact pass:** case-insensitive equality against `gov_name`.
    Single hit -> resolved. Multiple -> step 4.
-3. **Substring fallback:** case-insensitive regex against `gov_name`.
+3. **Substring fallback:** case-insensitive literal substring against
+   `gov_name` (metacharacters escaped).
    Single hit -> resolved (`match_method = "substring"`). Zero hits ->
    `status = "no_match"`. Multiple hits -> step 4.
 4. **Disambiguation:** if matches share one `govs_type`, pick the
@@ -58,7 +62,7 @@ inputs (`ambiguous` / `no_match`) appear only in the sidecar.
 }
 \examples{
 \dontrun{
-# Utility mode — exploratory regex lookup
+# Utility mode — exploratory substring lookup
 cog_gov_search("broward", state = "FL")
 
 # Basket mode — resolve a known cohort