Folder import makes two sequential passes over every file

`Playlist.entries(fromUrl:)` builds entries through `.async.compactMap { await MediaItem(url:) }`, which is strictly sequential. Then `autoSorted()` loops again, sequentially awaiting three metadata keys per entry.

Each `MediaItem(url:)` additionally creates a bookmark *and then immediately resolves it* (the standalone-access trade-up), so it's two filesystem round-trips per file before metadata even starts.

At a measured ~16ms/file (iPhone 15 Pro Max), 500 files is ~8 seconds. A `TaskGroup` with order reassembly would cut this substantially; it's the single biggest lever on import time.

Start at Playlist.entries(fromUrl:), MediaItem(url:), and autoSorted() to trace the two sequential passes and bookmark resolution. Evaluate TaskGroup-based order reassembly while preserving entry order, then measure importing 500 files to confirm the sequential import time is substantially reduced.

Written by the indexing model from the issue text.