feat: limit/offset on cog_gov_search() and cog_balances() (#57)

verbs were left materializing everything and slicing in R -- the pattern behind
the 2026-08-06 production incident. cog_gov_search() had no LIMIT at all, so an
unfiltered call returns the entire 40,336-row crosswalk.

Extracted the #39 machinery into R/pagination.R first (.validate_pagination(),
.paginate_sql(), .take_pagination_total()) rather than growing a third inline
copy: three definitions of what total_rows means is three places for it to
drift. Conflict refusals stay at the call sites because each verb's conflict
set differs. .verb_spendrev() now uses the shared helpers and is unchanged in
behaviour.

The empty-page fallback query is now passed as a thunk, so the unpaginated SQL
is only BUILT when an offset actually lands past the end instead of on every
paged call.

Two things #57 did not anticipate:

- cog_gov_search()'s ORDER BY was not a total order. population_acs DESC NULLS
  LAST leaves ties -- and the whole NULL block -- in scan order, so two requests
  can order them differently and a paged sweep duplicates one row while dropping
  another. Added canonical_govid as tiebreaker. Unpaginated output changes only
  in the relative order of already-tied rows.

- Basket mode returns one resolved row per requested name plus a sidecar
  covering all of them, so a page of it is not a page of anything the caller
  asked for. Refused with uscogdata_basket_pagination_conflict rather than
  silently ignoring the arguments.

Both default to NULL, so cog-api adopts them behind its existing formals()
probe with no lockstep deploy.

Suite: 1067 passed, 0 failed, 0 warnings (2 pre-existing live-corpus skips).

NEWS.md

@@ -19,6 +19,28 @@ at boot -- `getFromNamespace(".ensure_session", "uscogdata")()` followed by a ma
 `SET threads` -- depending both on a private name and on the session already being
 open.
 
+## `cog_gov_search()` and `cog_balances()` gain `limit`/`offset`
+
+Pagination arrived on `cog_spending()`/`cog_revenue()` in 0.3.0; the other two
+verbs were left materializing everything and slicing in R. Both now take
+`limit`/`offset` with the same semantics: `NULL` default, the page applied in
+SQL behind a deterministic `ORDER BY`, and the unpaginated count returned as a
+`total_rows` attribute computed by `COUNT(*) OVER()` in the same scan rather
+than a second query.
+
+`cog_gov_search()` had no `LIMIT` at all, which made it the one verb that
+returns the entire 40,336-row crosswalk when called with no filter.
+
+Two refusals rather than silent surprises:
+
+* `cog_balances(recipe = , limit = )` aborts with class
+  `uscogdata_recipe_pagination_conflict` -- a recipe's result comes from a
+  separate query that pagination is not wired into.
+* `cog_gov_search()` in basket mode (`length(name) > 1`) aborts with class
+  `uscogdata_basket_pagination_conflict`. Basket mode returns one resolved row
+  per requested name with a sidecar covering all of them; a page of that is not
+  a page of anything the caller asked for.
+
 ## Cohorts can be named by predicate, not just by id
 
 `cog_spending()`, `cog_revenue()` and `cog_balances()` gain optional `state`
@@ -67,6 +89,14 @@ A `govid`-named cohort's provenance is unchanged.
 
 ## Fixes
 
+* `cog_gov_search()` now orders by `population_acs DESC NULLS LAST,
+  canonical_govid`. **`population_acs` alone is not a total order** -- ties, and
+  the entire `NULLS LAST` block, came back in whatever order the scan produced.
+  That was invisible while every call returned the full result set, but it makes
+  a paged sweep unsound: two requests can order tied rows differently, so a row
+  is duplicated on one page and missing from the next. Unpaginated results are
+  unchanged except for the relative order of rows that were already tied.
+
 * An unknown `state` abbreviation now aborts with "Unknown state abbreviation"
   (class `uscogdata_unknown_state`) instead of base R's "subscript out of
   bounds". `.state_abbrev_to_fips` is a named character vector, so `[[` on an