Lift the in-memory ephemeral-folder library (synthetic-id store + folder
scan / metadata / CUE handling) out of the Tauri binary into a
frontend-agnostic qbz-library module (ADR-006), so both the Tauri and Slint
frontends consume identical logic. src-tauri/ephemeral_library now re-exports
it as a thin shim. Adds tracks_snapshot() / current_folder_path() for the
Slint consumer.
cc