find/build: emit item metadata (artists) instead of packing 'Artist — Title' into title strings

[moskovich]

Summary

Playlists built by ff-cli carry artist attribution nowhere except packed into title strings ("Casey Reas — Pre-Process"). Two consequences, both observed in production this week:

1. The FF1 tombstone overlay (Digital tombstones: museum label overlay on the Art Computer (feral-file#3452) ff-player#255) reads item.metadata for the artist line; ff-cli items have none, so every ff-cli playlist labels title-only.
2. Packing artists into titles corrupts attribution: a horizon(te)s item credited only "Zach Lieberman" because the em-dash convention can't express a collaboration (Velitchkova × Lieberman). Machines forcing humans to encode structured data in display strings is how this happens.

Proposal

find and build emit per-item metadata and keep titles bare:

"title": "Pre-Process",
"metadata": {
  "title": "Pre-Process",
  "artists": [{ "name": "Casey Reas", "id": "" }]
}

The artist data is already in hand at build time (Raster summaries carry artists[]; the FF indexer knows artists; playlistTitleFor currently joins them into the title string).

Blocked on

The exact field shape awaits the DP-1 inline-metadata standardization discussion on feral-file/ff-player#255 (proposal: inline carriage of the ref-manifest §4 metadata block). File now, implement when that lands — same wave as ffos-user#236.