feat(recommendation): Songs-like can include the seed artist's own music

Operator, 2026-09-10: "it should also be able to include music from the same
artist." Completes #3881 — the weights and pool landed in f367eeaa; this is
the eligibility half.

produceSeedMixes filtered the seed artist out entirely:

    // "Songs like X" excludes X's own songs.
    if !pgtypeUUIDEqual(c.Track.ArtistID, artistID) { ... }

That reads as obviously right and is not. The seed is a TRACK — the artist's
top-played one — and the tracks most likely to sound like it are usually the
rest of that artist's catalogue. The filter threw away the seed's nearest
neighbours, then reached FURTHER OUT to replace them. On the one surface
whose job is staying in a neighbourhood, that is backwards, and it worked
against the coherence tuning rather than with it.

Domination is bounded by the cap instead of by exclusion, which is the
distinction that makes this safe rather than a new problem:
capCandidatesByAlbumAndArtist already allows at most 3 tracks per artist in
a 25-track mix, so the seed artist gets 12% at most — a presence, not a
takeover. Without that bound this would just be the radio failure (#3882)
arriving on a different surface. The seed track itself still cannot appear;
it is passed to LoadCandidatesFromSimilarity as an exclusion.

Guarded end-to-end rather than by reading the source, for two reasons: the
check has to survive the filter returning in a different shape, and an
absence check would now match the comment that explains why the filter is
gone — rule 167's prose trap exactly. The test asserts both directions, that
at least one mix contains its seed artist and that none exceeds the cap.

Its falsification is by construction rather than by execution: under the
previous code every mix's own-artist count was necessarily zero, so the
assertion could not have passed. Running it needs Postgres, which is the
integration lane's job.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SQ31KQpYbStyK5y58UmPLH

internal/playlists/system.go

@@ -703,14 +703,26 @@ func produceSeedMixes(
 				"artist_id", uuidStringPL(artistID), "err", cerr)
 			continue
 		}
-		// "Songs like X" excludes X's own songs.
-		filtered := make([]recommendation.Candidate, 0, len(cands))
-		for _, c := range cands {
-			if !pgtypeUUIDEqual(c.Track.ArtistID, artistID) {
-				filtered = append(filtered, c)
-			}
-		}
-		tracks := pickTopN(filtered, userID, dateStr, now, systemMixLength)
+		// The seed artist's own songs are ELIGIBLE here, deliberately.
+		//
+		// This used to filter them out — "Songs like X excludes X's own
+		// songs" — which reads as obviously right and is not. The seed is a
+		// TRACK, and the tracks most likely to sound like it are usually the
+		// rest of that artist's catalogue; excluding them threw away the
+		// nearest neighbours of the very thing the mix is built around, and
+		// then reached further out to replace them. On a surface whose whole
+		// job is staying in one neighbourhood, that is backwards.
+		//
+		// Operator, 2026-09-10: "it should also be able to include music from
+		// the same artist."
+		//
+		// Domination is bounded by the cap rather than by exclusion, which is
+		// the distinction that makes this safe: capCandidatesByAlbumAndArtist
+		// inside pickTopN allows at most discoverMaxTracksPerArtist (3) of a
+		// 25-track mix — 12%, a presence rather than a takeover. The seed
+		// track itself still cannot appear; it is passed as an exclusion to
+		// LoadCandidatesFromSimilarity above.
+		tracks := pickTopN(cands, userID, dateStr, now, systemMixLength)
 		if len(tracks) == 0 {
 			continue
 		}