Merge origin/main (PR #8: corpus URL trailing-slash normalization) into feat/expenditure-concept

Local main was stale at fa40266 when this branch was created, so it was
missing 748ca4a. Merging rather than rebasing to preserve the reviewed
commit SHAs recorded in the SDD ledger.

R/config.R

@@ -21,7 +21,30 @@
   .uscogdata_defaults[[key]]
 }
 
-.resolve_url <- function() .cfg("url")
+#' Resolve the corpus URL, guaranteeing the trailing slash the package assumes.
+#'
+#' Every consumer builds locations by CONCATENATION -- `paste0(url,
+#' "manifest.json")` in manifest.R, `paste0(url, e$path)` in mirror.R, and the
+#' parquet glob in views.R -- and mirror.R:104 documents the invariant outright
+#' ('url ends in "/"'). Nothing enforced it, so a URL entered without the slash
+#' failed silently and misleadingly:
+#'
+#'   HTTPS -> ".../downloadmanifest.json"; the host answers with an HTML 404
+#'            page, which lands in the JSON parser as the lexical error
+#'            reported in issue #3 -- pointing the user at "login page / wrong
+#'            share" when the real cause was one missing character.
+#'   local -> ".../corpusdata/long/**/*.parquet" and a DuckDB "No files found".
+#'
+#' Normalizing here fixes every consumer at once, rather than each call site
+#' re-deriving the same invariant. An empty setting is passed through
+#' untouched so manifest.R's "not configured" guard still fires instead of the
+#' value degrading into a bare "/" filesystem root.
+#' @noRd
+.resolve_url <- function() {
+  url <- .cfg("url")
+  if (is.null(url) || !nzchar(url) || grepl("/$", url)) return(url)
+  paste0(url, "/")
+}
 
 .resolve_cache_dir <- function() {
   v <- .cfg("cache_dir")