docs: fix stale fixture description and Direct/Total vignette figures (M1, M2, M5)

M5: README.md described the bundled fixture as a "3.6 MB two-year slice
(2019 + 2020)"; it's now a 15 MB four-year slice (2011, 2012, 2019,
2020), matching the regenerated fixture and the vignette's own
description.

M2: total-spending.Rmd cited County 1.8% / City 0.8% intergovernmental-
to-Direct and County 91.6% L/M, all roughly 2x off against the bundled
fixture. Measured directly against the fixture (all 50 states, each of
its four years): County IG/Direct 3.4%-5.1%, City IG/Direct 2.6%-3.1%,
County L/M 43%-51% (all varying by year). State 17.2%, AL 7.6%,
national 11.6%, and City L/M 188.3% were re-checked and left as-is.

M1: the "Why Total = Direct + M + L" paragraph described money a local
government *receives* and the state "redistributing as M" -- backwards.
M and L are both the *queried* government's own payments *out*: M to
other local governments, L up to its state. Rewrote the explanation;
the conclusion and non-M2-flagged figures are unchanged.

README.md

@@ -47,9 +47,10 @@ explanation with worked examples.
 ### Testing
 
 The package ships a bundled fixture corpus at `inst/extdata/fixture_corpus/` —
-a 3.6 MB two-year slice (2019 + 2020) of the full corpus covering all 50
-states. `tests/testthat/setup.R` automatically points `USCOGDATA_URL` at this
-fixture, so the full test suite runs offline with no network dependency:
+a 15 MB four-year slice (2011, 2012, 2019, 2020) of the full corpus covering
+all 50 states. `tests/testthat/setup.R` automatically points `USCOGDATA_URL`
+at this fixture, so the full test suite runs offline with no network
+dependency:
 
 ```r
 devtools::test()   # uses bundled fixture, no credentials required