pub fn mem_cache_dir() -> PathBufExpand description
Resolve the global mem-cache directory.
Respects MEMSTEAD_MEM_CACHE if set — tests use this to point at a
tempdir without touching the real user-data directory. Otherwise
returns <data_dir>/memstead/mems on every platform (macOS / Linux /
Windows), so the CLI and the Memstead app resolve to the same path
without per-platform branching.
dirs::data_dir() is infallible on Tier-1 platforms; expect is
fine for an engine that only runs on systems with a resolvable home.