`beet modify` writes to file when no writeable tags changed

### Problem

Running this command:

```sh
$ beet modify -a onplayer=true queen
```

Led to this problem:

- Media files got updated mtime and the command took 25s, telling me it wrote to the files even though no writable media tags were changed. If I run with `-W` instead, the command takes <1s.
- As far as I can tell, all custom tags are filtered out in `library.py` when writing to media files, so `beet modify` can avoid writing entirely in this case. I modify this tag for example often to change what goes on my iPod and always use `-W` but would rather not have to worry about having the argument.

### Setup

* OS: NixOS
* Python version: 3.12.8
* beets version: 2.1.0
* Turning off plugins made problem go away (yes/no): no

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

```yaml
directory: /mnt/music
library: ~/music/beets-library.db
original_date: yes
format_album: $albumartist/[$year] $album ($release_year, $country, $label, $media)
format_item: $artist - $album - $disc.$track. $title
sort_album: albumartist+ year+ album+
per_disc_numbering: yes
import:
detail: yes
autotag: yes
copy: no
timid: yes
languages: en
incremental: yes
incremental_skip_later: no
write: yes
match:
preferred:
countries: ["US", "GB|UK", "JP"]
media: ["CD", "DigitalMedia|File", "Vinyl"]
original_year: yes
musicbrainz:
extra_tags: [year, catalognum, country, media]
paths:
albumtype:soundtrack: "0. Soundtracks/[$realyear] $album/%if{$multidisc,Disc %right{$disc,1}/}$track. $title"
albumtype:single: %the{$albumartist}/Singles/[$realyear]%if{$subartist,[$subartist]} $album/$track. $title
comp: "0. %first{$genre}/[$realyear]%if{$subartist,[$subartist]} $album%ifdef{atypes, $atypes}/%if{$multidisc,%if{$grouping,$grouplabel,$disclabel}/,%if{$grouping,$grouping/}}$track. $title"
default: %the{$albumartist}/[$realyear]%if{$subartist,[$subartist]} $album%ifdef{atypes, $atypes}/%if{$multidisc,%if{$grouping,$grouplabel,$disclabel}/,%if{$grouping,$grouping/}}$track. $title
item_fields:
mono: 1 if channels == 1 else 0
multidisc: 1 if disctotal > 1 else 0
# Vinyl records are usually split up into lettered sides, rather than numbered discs.
disclabel: ('Disc ' + str(disc))
grouplabel: ('Disc ' + str(disc) + '. ' + grouping)
# MF Doom is something special, with all of his alternate names
subartist: albumartist_credit if not comp and albumartist.lower() != albumartist_credit.lower() else ''
realyear: original_year if original_year != 0 and original_year < year else year
plugins: the albumtypes rewrite inline alternatives fetchart convert fromfilename badfiles lastgenre replaygain info types zero unimported lyrics random edit
aunique:
keys: albumartist album year albumtype
disambiguators: albumdisambig media country label catalognum
bracket: "()"
types:
release_year: int
zero:
fields: images acoustid_id acoustid_fingerprint asin script barcode comments
comments:
[
"^Exact ?Audio ?Copy",
^Visit http,
^Encoded by,
"^[Rr]ip(ped)? by",
^Track \d+$,
^EAC,
]
update_database: true
# Automatically resample all hi-fi FLACs down to CD quality on import
convert:
auto: yes
embed: no
max_bitrate: 1480
never_convert_lossy_files: yes
format: flac
formats:
flac24:
command: ffmpeg -y -i $source -acodec flac -af aresample=osf=s24:resampler=soxr:out_sample_rate=48000 $dest
extension: flac
flac: ffmpeg -y -i $source -acodec flac -af aresample=osf=s16:dither_method=triangular:resampler=soxr:out_sample_rate=48000 $dest
wav: ffmpeg -y -i $source -ar 44100 -sample_fmt s16 $dest
fetchart:
auto: yes
cautious: yes
cover_names: cover front folder album art
minwidth: 300
maxwidth: 2000
enforce_ratio: 3%
sources: filesystem coverart itunes
high_resolution: yes
deinterlace: yes
albumtypes:
types:
- ep: "EP"
- single: "Single"
- live: "Live"
- compilation: "Comp"
ignore_va: compilation
bracket: "()"
rewrite:
albumartist DANGERDOOM: MF DOOM
albumartist Viktor Vaughn: MF DOOM
albumartist King Geedorah: MF DOOM
albumartist Czarface & MF DOOM: MF DOOM
albumartist DOOM: MF DOOM
albumartist MF DOOM & MF Grimm: MF DOOM
albumartist Madvillain: MF DOOM
albumartist Efterklang & .*: Efterklang
albumartist The Jimi Hendrix Experience: Jimi Hendrix
albumartist Mychael Danna, DeVotchKa: DeVotchKa
albumartist Adrian Orange: Thanksgiving
albumartist The Velvet Underground & Nico: The Velvet Underground
albumartist kidkanevil & Daisuke Tanabe: Daisuke Tanabe
albumartist David Byrne & St. Vincent: David Byrne
albumartist filous & Daði Freyr: Daði Freyr
albumartist J\.U\.F: Gogol Bordello
albumartist Gorillaz feat.*: Gorillaz
albumartist Spacemonkeyz vs. Gorillaz: Gorillaz
albumartist Justin Timberlake & .*: Justin Timberlake
albumartist The Management: MGMT
albumartist Urban Thermo Dynamics: Mos Def
albumartist The Flaming Lips .*: The Flaming Lips
albumartist Prince and The Revolution: Prince
alternatives:
ipod:
directory: /run/media/snead/SHELBY_S IP/Music
query: "onplayer:true"
removable: true
albumart_maxwidth: 216
amelies_ipod:
directory: /run/media/snead/AD IPOD/Music
query: "ABBA , Beta Band , Beijing to Boston , Brave Irene , Buzzcocks , Chicano Batman , Freyr , Deltron 3030 , Dolly Parton , Hiatus Kaiyote , albumartist:Javelin , LCD Soundsystem , Lou Reed , MF DOOM , albumartist:Queen , Soft Hair , albumartist:'PJ Harvey' , Justin Timberlake album:Justified , Medscool , CZARFACE , Tatsuro Yamashita album:'For You' , Sibylle Baier , The Strokes , The Clash , Metallica"
removable: true
albumart_maxwidth: 216
# Replace regular slashes with the fraction slash! A trick to keep slashes in file names!! :)
path_sep_replace: "⁄"
replace:
'\s+$': ""
'\?': "¿"
'[\\/]': "⁄"
'^\.': _
'[\x00-\x1f]': _
'^\s+': ""
"^-": _
# Replace some unsupported characters with similar unicode ones (as long as Cantarell has them)
'\|': "¦"
":": ";"
"‐": "-"
'\*': "¤"
">": "›"
"<": "‹"
'\.\.+': "…"
# Replace smart single quotes with apostrophes. I think Cantarell was rendering smart single quotes weird?
'"': "″"
"’": "'"
# Fallback replacements
'[\?\.]+$': ""
'[<>\?]+': _
lastgenre:
auto: no
separator: "; "
count: 1
canonical: ~/.config/beets/genre-tree.yaml
min_weight: 20
force: no
source: album
whitelist: ~/.config/beets/genres.txt
replaygain:
backend: ffmpeg
auto: yes
unimported:
ignore_extensions: jpg png jpeg gif pdf JPG cue
ignore_subdirectories: Artwork Art Covers
lyrics:
auto: yes
synced: yes
```

Start with the `beet modify` command path and inspect the media-writing logic in `library.py`, where the issue says custom tags are filtered out. Reproduce the reported `beet modify -a onplayer=true queen` command and compare it with `-W`. Done means the command avoids writing media files, changing their mtime, or incurring the longer write time when no writable tags changed.

Written by the indexing model from the issue text.