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).

R/manifest.R

@@ -1,8 +1,19 @@
 # R/manifest.R
 
-#' Fetch manifest.json from URL, cache locally, validate TTL.
+#' Fetch manifest.json from URL (or read from a local fixture path),
+#' cache locally, validate TTL.
 #' @noRd
 .fetch_or_cache_manifest <- function(url, cache_dir) {
+  # Local fixture path: read manifest directly; skip cache/TTL plumbing so
+  # tests pick up regenerated manifests immediately.
+  if (.is_local_path(url)) {
+    local_manifest <- file.path(url, "manifest.json")
+    if (!file.exists(local_manifest)) {
+      cli::cli_abort("Local fixture has no manifest.json at {local_manifest}")
+    }
+    return(jsonlite::fromJSON(local_manifest, simplifyVector = FALSE))
+  }
+
   cache_path <- file.path(cache_dir, "manifest.json")
   ttl <- as.integer(.cfg("manifest_ttl_secs"))
 
@@ -19,6 +30,11 @@
   jsonlite::fromJSON(cache_path, simplifyVector = FALSE)
 }
 
+#' @noRd
+.is_local_path <- function(url) {
+  !grepl("^[a-zA-Z][a-zA-Z0-9+.-]*://", url)
+}
+
 #' @noRd
 .validate_schema <- function(manifest, expected_version) {
   if (manifest$schema_version != expected_version) {