`replaygain` plugin hangs for a few minutes doing nothing, before actually generating tags

### Problem
Command that generates replay gain tags hangs after this log for a couple of minutes:
```
Parsed query: AndQuery([TrueQuery()])
Parsed sort: NullSort()
```

Running this command in verbose (`-vv`) mode:

```sh
$ beet -vv replaygain -t 10 -w
user configuration: /home/xbt573/.config/beets/config.yaml
data directory: /home/xbt573/.config/beets
plugin paths:
fetchart: google: Disabling art source due to missing key
fetchart: lastfm: Disabling art source due to missing key
Sending event: pluginload
library database: /media/hdd/subsonic/beets.db
library directory: /media/hdd/subsonic/music
Sending event: library_opened
Parsed query: AndQuery([TrueQuery()])
Parsed sort: NullSort()
... 1-2 minutes of no output ...
-- log of successful replay gain generation --
```

### Setup

* OS: Fedora Linux 40
* Python version: 3.12.6
* beets version: 2.0.0 (installed from pipx)
* Turning off plugins made problem go away (yes/no): no

My configuration (output of `beet config`) is:

```yaml
lyrics:
bing_lang_from: []
sources: lrclib genius
synced: yes
fallback: ''
auto: yes
bing_client_secret: REDACTED
bing_lang_to:
google_API_key: REDACTED
google_engine_ID: REDACTED
genius_api_key: REDACTED
force: no
local: no
dist_thresh: 0.1
directory: /media/hdd/subsonic/music
# --------------- Main ---------------

library: /media/hdd/subsonic/beets.db

# --------------- Plugins ---------------

plugins: lyrics lastgenre fetchart embedart replaygain
paths:
default: $albumartist/$album%aunique{}/$track. $title
lastgenre:
fallback: yes
whitelist: yes
min_weight: 10
count: 1
canonical: no
source: album
force: yes
auto: yes
separator: ', '
prefer_specific: no
title_case: yes
fetchart:
maxwidth: 1200
auto: yes
minwidth: 0
quality: 0
max_filesize: 0
enforce_ratio: no
cautious: no
cover_names:
- cover
- front
- art
- album
- folder
sources:
- filesystem
- coverart
- itunes
- amazon
- albumart
- cover_art_url
store_source: no
high_resolution: no
deinterlace: no
cover_format:
google_key: REDACTED
google_engine: 001442825323518660753:hrh5ch1gjzm
fanarttv_key: REDACTED
lastfm_key: REDACTED
replaygain:
auto: no
backend: ffmpeg
overwrite: no
threads: 20
parallel_on_import: no
per_disc: no
peak: 'true'
targetlevel: 89
r128: [Opus]
r128_targetlevel: 84
replace:
'[\\/]': _
^\.: _
'[\x00-\x1f]': _
\s+$: ''
^\s+: ''
from_scratch: yes
embedart:
maxwidth: 0
auto: yes
compare_threshold: 0
ifempty: no
remove_art_file: no
quality: 0
```

Start by reproducing `beet -vv replaygain -t 10 -w` with the provided configuration and inspect the replaygain plugin entry point and its ffmpeg backend. Trace the pause after `Parsed sort: NullSort()` and compare it with the successful generation log. Done means replaygain starts without the multi-minute idle period while still generating the expected tags.

Written by the indexing model from the issue text.