fix(download): score a multi-disc rip as one album, and count tracks

Three faults in how candidates are shaped and scored, one commit because
they meet in the same completeness number.

Multi-disc albums were split in two.  Soulseek shares them as
Album/CD1 and Album/CD2, and candidates were grouped by the immediate
parent, so each disc became its own candidate titled "CD1": about half
complete, with an album title that could not match.  Such a release
essentially never cleared auto-pick.  AlbumDir groups a disc folder
under its parent, ParsePath takes the disc number from the folder (a
disc in the filename still wins), and collect keeps the disc folders in
staging, where flattened, disc 2's "01 Intro.flac" overwrote disc 1's.

A single-track request could never be served from Soulseek.  A track
search matches one file per folder, and the two-file floor that screens
out noise for an album screened out every result.  A recording request
takes one.

Completeness counted files.  Ten files against a ten-track album scored
full marks whether or not they were its tracks, and title fit is the
mean over the files that did align, so a folder where three titles
matched read as near-perfect on both.  Coverage is now counted in
aligned tracks, with the file count still setting the penalty for
extras.

Closes #270

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017HJiuc3ZZhxsPXz3ozTirT

backend/download/provider_slskd.go

@@ -70,8 +70,9 @@ const (
 	slskdTransferPoll = 3 * time.Second
 
 	// slskdMinFiles is the fewest audio files a folder needs before it
-	// is offered as a candidate.  Soulseek returns a lot of one-file
-	// noise for common queries.
+	// is offered as a candidate for an album.  Soulseek returns a lot of
+	// one-file noise for common queries.  A single-track request takes
+	// one (see minFilesFor).
 	slskdMinFiles = 2
 
 	// slskdHTTPTimeout bounds one API call.
@@ -330,7 +331,24 @@ func (s *slskd) Search(ctx context.Context, dl Download) ([]Candidate, error) {
 		)
 	}()
 
-	return s.candidatesFrom(search), nil
+	return s.candidatesFrom(search, minFilesFor(dl)), nil
+}
+
+// minFilesFor is the fewest audio files a folder must offer to be a
+// candidate for this request.
+//
+// Soulseek answers a search with the files that match it, not with the
+// folders they sit in.  An album query matches every file in the album's
+// folder, because the folder name carries the terms; a *track* query
+// usually matches one file per folder.  The two-file floor that filters
+// out one-file noise for an album therefore filtered out every result
+// for a track, and a single-track request could never be served here.
+func minFilesFor(dl Download) int {
+	if dl.RecordingMBID != "" {
+		return 1
+	}
+
+	return slskdMinFiles
 }
 
 // awaitSearch polls until the search completes or the budget runs out.
@@ -371,8 +389,9 @@ func (s *slskd) awaitSearch(
 	return last, nil
 }
 
-// candidatesFrom groups a search's responses into candidates.
-func (s *slskd) candidatesFrom(search slskdSearch) []Candidate {
+// candidatesFrom groups a search's responses into candidates, dropping
+// folders with fewer than minFiles audio files.
+func (s *slskd) candidatesFrom(search slskdSearch, minFiles int) []Candidate {
 	out := make([]Candidate, 0, len(search.Responses))
 
 	for _, resp := range search.Responses {
@@ -400,7 +419,7 @@ func (s *slskd) candidatesFrom(search slskdSearch) []Candidate {
 				total += f.Size
 			}
 
-			if audio < slskdMinFiles {
+			if audio < minFiles {
 				continue
 			}
 
@@ -421,13 +440,15 @@ func (s *slskd) candidatesFrom(search slskdSearch) []Candidate {
 	return out
 }
 
-// groupByFolder buckets a peer's files by their containing directory.
+// groupByFolder buckets a peer's files by the album directory they sit
+// in — the containing directory, or the one above it for a disc folder
+// (see AlbumDir), so a multi-disc rip is one candidate and not two.
 func groupByFolder(files []slskdFile) map[string][]slskdFile {
 	out := map[string][]slskdFile{}
 
 	for _, f := range files {
-		norm := strings.ReplaceAll(f.Filename, `\`, "/")
-		out[path.Dir(norm)] = append(out[path.Dir(norm)], f)
+		dir := AlbumDir(f.Filename)
+		out[dir] = append(out[dir], f)
 	}
 
 	return out
@@ -817,7 +838,13 @@ func (s *slskd) collect(c Candidate, dst string) (Result, error) {
 			continue
 		}
 
+		// A multi-disc candidate keeps its disc folders in staging.
+		// Flattened, disc 2's "01 Intro.flac" overwrites disc 1's, and
+		// the importer loses the folder it reads the disc number from.
 		target := filepath.Join(dst, base)
+		if _, ok := discFolder(folder); ok {
+			target = filepath.Join(dst, folder, base)
+		}
 
 		if err := movePath(src, target); err != nil {
 			return Result{}, fmt.Errorf("collect %s: %w", base, err)