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/session.R

@@ -0,0 +1,43 @@
+# R/session.R
+
+#' Internal: open session, register views, cache manifest.
+#' Not exported. Called lazily by verbs via .ensure_session().
+#' @noRd
+cog_open <- function(url = .resolve_url(),
+                     cache_dir = .resolve_cache_dir()) {
+  if (!dir.exists(cache_dir)) dir.create(cache_dir, recursive = TRUE)
+
+  con <- DBI::dbConnect(duckdb::duckdb())
+  DBI::dbExecute(con, "INSTALL httpfs; LOAD httpfs;")
+
+  manifest <- .fetch_or_cache_manifest(url, cache_dir)
+  .validate_schema(manifest, expected_version = 2L)
+  .validate_scope(manifest)
+
+  .register_views(con, url, manifest)
+
+  .uscogdata_env$con       <- con
+  .uscogdata_env$manifest  <- manifest
+  .uscogdata_env$url       <- url
+  .uscogdata_env$cache_dir <- cache_dir
+
+  invisible(con)
+}
+
+#' @noRd
+.ensure_session <- function() {
+  if (is.null(.uscogdata_env$con) ||
+      !DBI::dbIsValid(.uscogdata_env$con)) {
+    cog_open()
+  }
+  .uscogdata_env$con
+}
+
+#' @noRd
+cog_close <- function() {
+  if (!is.null(.uscogdata_env$con) && DBI::dbIsValid(.uscogdata_env$con)) {
+    DBI::dbDisconnect(.uscogdata_env$con, shutdown = TRUE)
+  }
+  .uscogdata_env$con      <- NULL
+  .uscogdata_env$manifest <- NULL
+}