refactor(v2): inline library scan and metadata _impl delegates

Inlines bodies of library_scan_impl, library_stop_scan_impl,
library_scan_folder_impl, library_clear_impl, library_update_album_metadata_impl,
library_write_album_metadata_to_files_impl, and library_refresh_album_metadata_from_files_impl
from library/commands.rs directly into the v2 wrappers in commands_v2/legacy_compat.rs.
Moves their helpers (process_cue_file, apply_sidecar_override_if_present,
compute_track_artist_match, normalize_library_path) alongside the inlined bodies.

Deletes the corresponding legacy #[tauri::command] _impl definitions and
the dead public command siblings (library_scan, library_stop_scan,
library_scan_folder, library_clear, library_update_album_metadata,
library_write_album_metadata_to_files, library_refresh_album_metadata_from_files)
that were only ever called from the deleted _impl variants.
Updates library/mod.rs to drop the corresponding re-exports.

Step 8 of legacy cleanup, group A.

cc

src-tauri/src/library/mod.rs

@@ -86,15 +86,12 @@ pub use state::{
     LibraryAlbumTrackMetadataUpdate, LibraryState,
 };
 pub use commands::{
-    library_add_folder, library_cleanup_missing_files, library_clear_impl,
+    library_add_folder, library_cleanup_missing_files, library_fetch_missing_artwork,
-    library_fetch_missing_artwork, library_get_album_tracks, library_get_artists,
+    library_get_album_tracks, library_get_artists, library_get_folders,
-    library_get_folders, library_get_folders_with_metadata, library_get_scan_progress,
+    library_get_folders_with_metadata, library_get_scan_progress, library_get_stats,
-    library_get_stats, library_get_thumbnail, library_get_thumbnails_cache_size,
+    library_get_thumbnail, library_get_thumbnails_cache_size, library_get_tracks_by_ids,
-    library_get_tracks_by_ids, library_refresh_album_metadata_from_files_impl,
+    library_search, playlist_get_custom_order, playlist_get_tracks_with_local_copies,
-    library_scan_folder_impl, library_scan_impl, library_search, library_stop_scan_impl,
+    playlist_has_custom_order, update_playlist_folder,
-    library_update_album_metadata_impl, library_write_album_metadata_to_files_impl,
-    playlist_get_custom_order, playlist_get_tracks_with_local_copies, playlist_has_custom_order,
-    update_playlist_folder,
 };
 
 use std::path::Path;