fix(media): a sidecar named "episode.mkv.srt" anchors, and loses the container word

The first run against the real library died in the anchor lookup: a subtitle
whose own name carries the media extension matches BOTH prefix forms of its
video, and the "keep the longest" comparison read the destination slot of the
tuple instead of the length slot. The fixture only ever matched one prefix, so
the seam was green over a crash.

The tail the sidecar keeps is now stripped of that container word: Jellyfin and
Plex both read "Show S01E01.forced.srt", not "Show S01E01.mkv.forced.srt", and
an nfo still carries the media extension because that one is constructed, not
parsed.

tests/media_shows.bats

@@ -54,6 +54,7 @@ setup() {
     # trailer: four things that have to end up in four different places.
     at "$SEASON/The.Examples.S01E01.Pilot.1080p.x265.mkv" 9000
     at "$SEASON/The.Examples.S01E01.Pilot.1080p.x265.en.srt" 200
+    at "$SEASON/The.Examples.S01E01.Pilot.1080p.x265.mkv.forced.srt" 250
     at "$SEASON/folder.jpg" 300
     at "$SEASON/Trailers/teaser.mkv" 1000
     at "$SEASON/tvshow.nfo" 400
@@ -143,6 +144,12 @@ PY
         = "$LIB/The Examples (2010)/Season 01/The Examples S01E01.en.srt" ]
 }
 
+@test "a subtitle that carries the container's word in its own name still follows the episode" {
+    plan
+    [ "$(dst "$SEASON/The.Examples.S01E01.Pilot.1080p.x265.mkv.forced.srt")" \
+        = "$LIB/The Examples (2010)/Season 01/The Examples S01E01.forced.srt" ]
+}
+
 @test "a season image keeps its own name; a series nfo keeps the series folder" {
     plan
     [ "$(dst "$SEASON/folder.jpg")" = "$LIB/The Examples (2010)/Season 01/folder.jpg" ]