feat: package skeleton — DESCRIPTION, NAMESPACE, session/manifest/cache/views

Minimal skeleton for uscogdata v0.1. Internal session layer with lazy
cog_open(), manifest fetch+validate+cache, view registration placeholder.
Depends on DuckDB >=1.0, httr2, jsonlite. inst/schemas/provenance-v1.json
ships the structured provenance JSON Schema.

R/manifest.R

@@ -0,0 +1,40 @@
+# R/manifest.R
+
+#' Fetch manifest.json from URL, cache locally, validate TTL.
+#' @noRd
+.fetch_or_cache_manifest <- function(url, cache_dir) {
+  cache_path <- file.path(cache_dir, "manifest.json")
+  ttl <- as.integer(.cfg("manifest_ttl_secs"))
+
+  needs_fetch <- !file.exists(cache_path) ||
+                 difftime(Sys.time(), file.info(cache_path)$mtime, units = "secs") > ttl
+
+  if (needs_fetch) {
+    resp <- httr2::request(paste0(url, "manifest.json")) |>
+      httr2::req_error(is_error = function(r) httr2::resp_status(r) >= 400) |>
+      httr2::req_perform()
+    writeLines(httr2::resp_body_string(resp), cache_path)
+  }
+
+  jsonlite::fromJSON(cache_path, simplifyVector = FALSE)
+}
+
+#' @noRd
+.validate_schema <- function(manifest, expected_version) {
+  if (manifest$schema_version != expected_version) {
+    cli::cli_abort(c(
+      "Corpus schema version mismatch.",
+      x = "Package expects schema_version = {expected_version}; corpus has {manifest$schema_version}.",
+      i = "Update uscogdata (install.packages or pak::pkg_install) or re-publish corpus."
+    ))
+  }
+}
+
+#' @noRd
+.validate_scope <- function(manifest) {
+  included <- manifest$scope$gov_types_included
+  .uscogdata_env$scope_included <- included
+  invisible(NULL)
+}
+
+`%||%` <- function(a, b) if (is.null(a) || (length(a) == 1 && is.na(a))) b else a