Internal helper returning the root directory for the ISTAT metadata cache
(codelists.rds, flussi_istat.rds, codelist_metadata.rds,
dataset_codelist_map.rds, data_download_log.rds). The directory is
resolved from the ISTATLAB_CACHE_DIR environment variable via
Sys.getenv("ISTATLAB_CACHE_DIR", unset = "meta"), so multiple projects
can share a single cache. When the variable is unset (or empty), the
historical default "meta" (relative to the working directory) is used,
preserving backward compatibility.