docs: total-spending vignette + README section on Direct vs Total

Task 7 (final) of the expenditure_concept plan. The vignette leads with the
two archetype questions -- a single government's own trend (either concept
works, held fixed across years) vs a cross-government rollup (direct only,
with the refusal error from cog_geographic_rollup() shown and explained) --
walked through with code that runs against the bundled fixture corpus
(years 2011/2012/2019/2020, substituting for "2017 vs today"). Explains the
double-counting mechanism (a state's M44 payment to a county is the same
dollar as the county's own E44/F44), why Total = Direct + M + L rather than
Direct + M, and the composition rules (expenditure_concept is orthogonal to
basis, mutually exclusive with recipe). README gets a short pointer section
with the one-line rule.

README.md

@@ -25,6 +25,23 @@ package implements.
 - `USCOGDATA_CACHE_DIR` — optional override for the manifest cache directory
 - `USCOGDATA_MANIFEST_TTL_SECS` — optional manifest re-fetch TTL (default 3600)
 
+## Direct vs Total spending
+
+`cog_spending(..., expenditure_concept = c("direct", "total"))` controls
+whose spending a result counts. `"direct"` (the default) is a government's
+own current operations, capital outlay, and other direct spending. `"total"`
+additionally adds in the intergovernmental legs — money it hands to other
+governments to spend on its behalf — which is meaningful for describing one
+government's own budget over time, but double-counts when summed across
+governments (a state's payment to a county is the same dollar the county
+reports as its own direct spending).
+
+**Rule of thumb: any figure that spans more than one government uses
+`direct`.** `cog_geographic_rollup()` and `cog_peer_compare()` enforce this
+by refusing `expenditure_concept = "total"`. See
+`vignette("total-spending", package = "uscogdata")` for the full
+explanation with worked examples.
+
 ## Developer notes
 
 ### Testing