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/rank_test.go

@@ -282,28 +282,32 @@ func TestCompleteness(t *testing.T) {
 
 	tests := []struct {
 		name     string
-		got      int
+		aligned  int
+		audio    int
 		want     int
 		minScore float64
 		maxScore float64
 	}{
-		{"exact", 10, 10, 1.0, 1.0},
-		{"half missing", 5, 10, 0.49, 0.51},
-		{"one bonus track", 11, 10, 0.95, 1.0},
-		{"double", 20, 10, 0.74, 0.76},
-		{"nothing", 0, 10, 0, 0},
-		{"no expectation", 5, 0, 0.5, 0.5},
+		{"exact", 10, 10, 10, 1.0, 1.0},
+		{"half missing", 5, 5, 10, 0.49, 0.51},
+		{"one bonus track", 10, 11, 10, 0.95, 1.0},
+		{"double", 10, 20, 10, 0.74, 0.76},
+		{"nothing", 0, 0, 10, 0, 0},
+		{"no expectation", 0, 5, 0, 0.5, 0.5},
+		// Ten files are not ten tracks: three that align are three.
+		{"right count, wrong tracks", 3, 10, 10, 0.29, 0.31},
+		{"files that align to nothing", 0, 10, 10, 0, 0},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
 			t.Parallel()
 
-			got := completeness(tt.got, tt.want)
+			got := completeness(tt.aligned, tt.audio, tt.want)
 			if got < tt.minScore || got > tt.maxScore {
 				t.Errorf(
-					"completeness(%d, %d) = %f, want in [%f, %f]",
-					tt.got, tt.want, got, tt.minScore, tt.maxScore,
+					"completeness(%d, %d, %d) = %f, want in [%f, %f]",
+					tt.aligned, tt.audio, tt.want, got, tt.minScore, tt.maxScore,
 				)
 			}
 		})