`beet ls` regex queries do not match individual values in multi-valued `genres`

With Beets 2.13.1, regex queries on the multi-valued genres field do not appear to match individual values.

For example, I have tracks with:
```
genres: Electro; Electroclash; Synth-pop
```

The field is correctly populated and displayed:
```
$ beet ls -f '$artist - $title :: $genres' | grep Kittin
Miss Kittin & The Hacker - Life On MTV :: Electro; Electroclash; Synth-pop
Miss Kittin & The Hacker - Frank Sinatra :: Electro; Electroclash; Synth-pop
```

However, a regex matching an individual genre returns no results:
```
$ beet ls 'genres::^Electro$'
```
No output.

```
beet ls 'genres::(^|; )Electro(; |$)'
```
Also outputs nothing.

### Setup
Debian 13
Python 3.13.5
Beets 2.13.1
Bash

The `genres` field is populated with the Discogs plugin.

Start by reproducing the two `beet ls` regex queries against a library containing the shown multi-valued `genres` values, then trace how the CLI query is applied to individual values. Done means regex matching works for an individual genre and regression coverage verifies both example queries.

Written by the indexing model from the issue text.