fix: plumb albumArtist through Swift metadata embedBatch API (issue #74)

[ShadyUnderLight]

Summary

Plumb albumArtist through Swift metadata embedBatch API (Issue #74).

Problem

embed_metadata.py already supports reading item.get("albumArtist") with format mappings ready:

• FLAC to ALBUMARTIST
• MP3 to TPE2
• M4A to ©aART

But the Swift embedBatch API lacks the albumArtist parameter, breaking the data flow at the business layer.

Changes

1. Library/MetadataEmbedder.swift

   • Add albumArtist: String? parameter to MetadataWriter.embedBatch protocol
   • PythonMetadataAdapter.embedBatch serializes albumArtist into JSON payload
   • MetadataEmbedder.embedBatch passes through the parameter

2. Library/TrackSplitterEngine.swift

   • Pass albumArtist: performer when calling embedBatch (CUE PERFORMER as album-level artist source)

3. Tests/MetadataEmbeddingTests.swift

   • Update all embedBatch calls with albumArtist parameter
   • Add FLAC albumartist readback verification
   • Add MP3 TPE2 readback verification
   • Add M4A ©aART readback verification

4. docs/METADATA_MATRIX.md

   • Add album artist row to output format table
   • Update Swift API signature, mark albumArtist as reachable
   • Remove "not plumbed" notes

Verification

• Swift build succeeds
• 108 tests pass (6 metadata embedding tests now include albumArtist assertions)