feat: cog_spending + cog_revenue + cog_explain

Three core query verbs over the spending_annotated / revenue_annotated
DuckDB views. Each verb accepts vector govid, vector years, optional
category filter, per_capita flag, and adjust_to_year for CPI-U
real-dollar conversion (bundled index).

Amounts are returned in full USD (SUM(amt) * 1000) so callers can
freely rescale to millions/billions. The $1,000s -> $USD conversion
is recorded in provenance$transformations$units_conversion.

Every result carries an attr(., "provenance") list matching
inst/schemas/provenance-v1.json. cog_explain() prints the structured
form via cli or returns the raw list for MCP/JSON consumers.

Also: .fetch_or_cache_manifest() now handles local fixture paths so
tests can point USCOGDATA_FIXTURE_URL at the pipeline publish_cache/
without a working HTTP server.

Tests: 80 pass / 0 fail. devtools::check() 0E/0W/2N (both notes
pre-existing / environmental).

man/cog_explain.Rd

@@ -0,0 +1,23 @@
+% Generated by roxygen2: do not edit by hand
+% Please edit documentation in R/explain.R
+\name{cog_explain}
+\alias{cog_explain}
+\title{Explain a verb result's provenance}
+\usage{
+cog_explain(result, format = c("print", "list"))
+}
+\arguments{
+\item{result}{A tibble returned by a `cog_*` verb.}
+
+\item{format}{`"print"` (default) for a human-readable cli summary;
+returns `result` invisibly for chaining. `"list"` returns the raw
+provenance list (identical to `attr(result, "provenance")`).}
+}
+\value{
+Either `result` (invisibly) or the provenance list.
+}
+\description{
+Prints the structured provenance attached to a tibble returned by any
+`cog_*` verb, or returns it as a list for downstream use (MCP tools,
+dashboards, JSON export).
+}