Playlist: Add bulk track metadata editing

[scruffian]

What?

Adds a multi-selection inspector panel for Playlist Track blocks so artist, album, track image, and alternative text can be applied to all selected tracks.

Why?

Playlist tracks could only be edited one at a time, which made shared album, artist, and artwork metadata repetitive for multi-track playlists.

How?

The editor detects multi-selected core/playlist-track blocks with the same playlist parent and calls updateBlockAttributes for the selected client IDs while hiding duplicated single-track inspector panels.

Testing Instructions

1. Open a post or page.
2. Insert a Playlist block with multiple audio tracks.
3. Select multiple Playlist Track blocks.
4. In the sidebar, change the selected tracks' artist, album, track image, and alternative text.
5. Confirm the selected tracks update together.

Testing Instructions for Keyboard

1. Use keyboard block navigation to select multiple Playlist Track blocks.
2. Tab to the sidebar controls and edit Artist, Album, Track image, and Alternative text.
3. Confirm the changes apply to the selected tracks.

Screenshots or screencast

Not included.

Use of AI Tools

This PR was authored with assistance from OpenAI Codex; the generated changes were reviewed and tested locally.

[github-actions]

Size Change: +412 B (+0.01%)

Total Size: 7.72 MB

📦 View Changed

Filename	Size	Change
build/scripts/block-library/index.min.js	357 kB	+412 B (+0.12%)

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