fix: make loadTrack reconcile track metadata

[jeryj]

Summary

Make loadTrack() a complete in-place track swap for per-track metadata.

Previously, consumers could update the audio source with loadTrack(), but artist and artwork DOM could only be updated when the relevant elements already existed. If a new track added or removed artist/artwork metadata, consumers had to destroy and recreate the player to get the DOM into the right shape.

This updates loadTrack() to reconcile those fields in place:

• add, update, or remove artist text
• add, update, or remove artwork
• update artwork alt text
• keep artwork alt text in sync when artworkAlt is supplied

Why

This lets consumers switch tracks without tearing down the player instance when metadata presence changes, avoiding unnecessary DOM churn and visible layout shifts.

Testing

• npm test
• npm run build

Co-authored-by: Codex codex@openai.com