Flag slave-only songs whose album or artist the master has

Scans now mark slave-only songs as missing from the master when the master
has their album folder or at least their artist folder. A new Missing from
master filter in Review lists them, split by album and artist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -21,7 +21,7 @@ path on both sides. The fallbacks below catch small differences, and a scan warn
 | Same title after ignoring track numbers/punctuation (`01 - Song!.flac` vs `Song.mp3`) | Status `similar` (similar title). Manual review. |
 | Same top-level folder and title, but a **different subfolder** | Status `similar` (same top folder, other subfolder). Manual review. |
 | *(opt-in)* Approximately the same song in another folder, e.g. a **single vs. the album version** or a **disc folder** | Status `similar` (approximate match), with the reason shown. Manual review. |
-| Only exists in the slave library | Status `unmatched`. Never touched. |
+| Only exists in the slave library | Status `unmatched`. Never touched. Flagged as *missing from master* when the master has the album or artist. |
 | Already the same inode | Status `linked`. |
 
 **Approximate matching** is an opt-in setting on each pair. It finds songs whose file name or folder doesn't line up exactly with the
@@ -86,6 +86,13 @@ recommended to keep (the album version, then one already linked to the master, t
 **Remove** moves the selected copies into the quarantine after re-checking that each is unchanged and that another copy is still there;
 one copy of every song is always kept, emptied folders are removed, dry run applies, and every removal can be restored from History.
 
+**Missing from master.** The Review tab's **Missing from master** filter lists slave-only songs whose artist the master library
+has, so you can complete the master. *Album in master* means the master has the song's album folder but not the song (the slave has
+`Caravan Palace/Robot Face/Waterguns.mp3`, the master has `Caravan Palace/Robot Face/` without it); *only the artist in master* means the
+whole album is missing. Folders are compared ignoring case, disc folders and release labels such as *Deluxe Edition*, and each row names
+the master folder it found. Scans fill this in, nothing is changed, and the CSV export includes it. With approximate matching off, songs
+filed under a slightly different name or release also land here.
+
 **Listening in build mode.** Every song in the Build tab has a play button. A mini-player along the bottom of the page plays the
 folder in order, with seeking, previous/next, volume and space to pause. Songs are streamed from the master library as they are
 (nothing is converted), so they play when the browser supports the format: FLAC, MP3, AAC/M4A, Ogg/Opus and WAV in current browsers,