//! Five commands: `pick_folder` → `{ uri, path }` (path null when the tree
//! is not on a volume with a path), `all_files_access` → `{ granted }`,
//! `request_all_files_access`, which opens the system screen, `pick_image`
//! → `{ path }` (a photo, copied into the app's cache), and `pick_file` →
//! `{ path, name, size, copied }` — the document picker with the caller's
//! own MIME filter.
//!
//! `pick_file` copies as little as it can, because an image is small and an
//! audio file is not. A document already on this phone's storage hands its
//! own path over as it lies, so picking a 300 MB file costs nothing; one
//! with no path of its own is copied into `cacheDir/picked` under its own
//! display name, and only as far as `maxCopyBytes` allows — zero refuses
//! the copy outright. Those copies are swept at the start of every pick, so
//! a path from one pick is gone by the next: read it in the same breath.
//! The display name travels beside the path because it is load-bearing —
//! the playlist parser picks its format off the extension and titles the
//! draft from the stem.
//!
//! On the desktop the plugin is registered so the permission set resolves,
//! and every command answers "not here".