feat(explore): carry the local row id on a top result

`TopResult` was the one projection here that shipped `inLibrary` and no
local id, so the top-results cards had no choice but to read the weaker
flag. Every sibling model — `MBArtist`, `MBReleaseGroup`, `MBRecording`
— already carries `LocalID`, and the candidate builders had the value
in hand at every construction site.

`LocalID` is set and cleared by a test against `audio_files`, so it
means "there is something of mine here". `InLibrary` is written by the
same pass but is a one-way ratchet the prune can only clear alongside a
local id; it stays for scoring, which is where an approximate answer is
fine.

backend/explore/explore.go

@@ -2212,6 +2212,7 @@ func (e *Service) gatherTopCandidates(
 				ArtistType: a.Type,
 				Country:    a.Country,
 				InLibrary:  a.InLibrary,
+				LocalID:    a.LocalID,
 			},
 			category:     "artist",
 			qualityScore: quality,
@@ -2243,6 +2244,7 @@ func (e *Service) gatherTopCandidates(
 				ArtistType: a.Type,
 				Country:    a.Country,
 				InLibrary:  a.InLibrary,
+				LocalID:    a.LocalID,
 			},
 			category:     "artist",
 			qualityScore: quality,
@@ -2275,6 +2277,7 @@ func (e *Service) gatherTopCandidates(
 				PrimaryType:  rg.PrimaryType,
 				Year:         year,
 				InLibrary:    rg.InLibrary,
+				LocalID:      rg.LocalID,
 			},
 			category:     "release_group",
 			qualityScore: quality,
@@ -2320,6 +2323,7 @@ func (e *Service) gatherTopCandidates(
 				PrimaryType:  rg.PrimaryType,
 				Year:         year,
 				InLibrary:    rg.InLibrary,
+				LocalID:      rg.LocalID,
 			},
 			category:     "release_group",
 			qualityScore: quality,
@@ -2347,6 +2351,7 @@ func (e *Service) gatherTopCandidates(
 				CAAReleaseMBID: r.CAAReleaseMBID,
 				ReleaseName:    r.ReleaseName,
 				InLibrary:      r.InLibrary,
+				LocalID:        r.LocalID,
 			},
 			category:     "recording",
 			qualityScore: quality,
@@ -2403,6 +2408,7 @@ func (e *Service) gatherTopCandidates(
 				CAAReleaseMBID: r.CAAReleaseMBID,
 				ReleaseName:    r.ReleaseName,
 				InLibrary:      r.InLibrary,
+				LocalID:        r.LocalID,
 			},
 			category:     "recording",
 			qualityScore: quality,
@@ -2425,6 +2431,7 @@ func (e *Service) gatherTopCandidates(
 					ArtistType: m.ArtistType,
 					Country:    m.Country,
 					InLibrary:  m.InLibrary || m.LocalArtistID > 0,
+					LocalID:    m.LocalArtistID,
 				},
 				category:     "artist",
 				qualityScore: quality,
@@ -2445,6 +2452,7 @@ func (e *Service) gatherTopCandidates(
 					PrimaryType:  m.PrimaryType,
 					Year:         year,
 					InLibrary:    m.InLibrary || m.LocalReleaseGroupID > 0,
+					LocalID:      m.LocalReleaseGroupID,
 				},
 				category:     "release_group",
 				qualityScore: quality,
@@ -2459,6 +2467,7 @@ func (e *Service) gatherTopCandidates(
 					ArtistMBID:   m.ArtistMBID,
 					Length:       m.Duration,
 					InLibrary:    m.InLibrary || m.LocalRecordingID > 0,
+					LocalID:      m.LocalRecordingID,
 				},
 				category:     "recording",
 				qualityScore: quality,

backend/explore/types.go

@@ -41,7 +41,16 @@ type TopResult struct {
 	ReleaseGroupMBID string `json:"releaseGroupMbid,omitempty"`
 	ReleaseName      string `json:"releaseName,omitempty"`
 	// Library status — populated from index cross-reference columns.
-	InLibrary bool `json:"inLibrary"`
+	//
+	// LocalID is the one the cards read.  It is the local row behind
+	// this entity — an album, a file, an artist — and it is set and
+	// cleared by a test against `audio_files`, so it means "there is
+	// something of mine here".  InLibrary is written by the same pass
+	// but is a one-way ratchet the prune can only clear alongside a
+	// local id, so it is the weaker of the two and stays for scoring
+	// (`fwInLibrary`), which is where an approximate answer is fine.
+	InLibrary bool  `json:"inLibrary"`
+	LocalID   int64 `json:"localId,omitempty"`
 }
 
 // MBArtist is a Wails-friendly projection of a MusicBrainz artist.