feat: split unmatched listening by whether the library holds the title

"Unmatched by an artist the library holds" was presented as the matcher's
misses. On real data it is not: owning one album by an artist says nothing about
owning a particular single of theirs, and most of that figure turned out to be
drum and bass tracks streamed but never bought.

Split it in two. A title the library holds under some other artist is an
attribution disagreement -- a remixer credited as the artist, a guest billed as
one -- and is a genuine miss worth fixing; the report now names the artist the
library files it under, which is the information needed to judge it. A title the
library does not hold at all under any artist was never bought, and no
improvement to matching will conjure it.

The distinction matters beyond presentation. That figure is the gate on the
cull, and a gate computed from a number that overstates the failure rate blocks
work that is actually safe to do.

README.md

@@ -109,10 +109,18 @@ matcher. The line to watch is:
 unmatched by an artist the library holds: N pairs, M plays
 ```
 
-That is a track that was played, sitting beside a file it should have matched.
-Those are the matcher's real misses, and every one is a candidate for being
-wrongly called cold in stage four. The report lists the worst fifteen by play
-count so they can be eyeballed.
+That is a track that was played by an artist the library holds. It is then
+split again, because owning an artist is a weak proxy for owning a track:
+
+- **the title exists under another artist** — an attribution disagreement, a
+  remixer or a guest billed as the artist. These are the genuine misses, and
+  each is a candidate for being wrongly called cold in stage four. The report
+  names the artist the library files them under.
+- **the title is nowhere in the library** — never bought. No amount of matching
+  conjures a file that does not exist.
+
+Counting both as matcher failures overstates the problem and would over-block
+the cull. The report lists the worst of each by play count.
 
 ## How the ingest works