feat: rework !similar to build and play discovery playlists

Default !similar now discovers similar artists/tracks, resolves each
against YouTube in parallel via ThreadPoolExecutor, fades out current
playback, and starts the new playlist. Old display behavior moves to
!similar list subcommand.

New helpers: _search_queries() normalizes Last.fm/MB results into search
strings, _resolve_playlist() resolves queries to _Track objects in
parallel. Falls back to display mode when music plugin not loaded.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/USAGE.md

@@ -1791,19 +1791,22 @@ key is configured; falls back to MusicBrainz automatically (no key
 required).
 
 ```
-!similar              Similar to currently playing track
-!similar <artist>     Similar artists to named artist
-!similar play         Queue a random similar track
-!similar play <artist> Queue a similar track for named artist
+!similar              Discover + play similar to current track
+!similar <artist>     Discover + play similar to named artist
+!similar list         Show similar (display only)
+!similar list <artist> Show similar for named artist
 !tags                 Genre tags for currently playing artist
 !tags <artist>        Genre tags for named artist
 ```
 
+- Default `!similar` builds a discovery playlist: finds similar artists/tracks,
+  resolves each against YouTube in parallel, fades out current playback, and
+  starts the new playlist
+- `!similar list` shows results without playing (old default behavior)
 - When an API key is set, Last.fm is tried first for richer results
 - When no API key is set (or Last.fm returns empty), MusicBrainz is
   used as a fallback (artist search -> tags -> similar recordings)
-- `!similar play` picks a random result and delegates to `!play`
-  (searches YouTube for the artist + title)
+- Without the music plugin loaded, `!similar` falls back to display mode
 - MusicBrainz rate limit: 1 request/second (handled automatically)
 
 Configuration (optional):