fix(artwork): retry remaining fallbacks when the first one fails to open

Review feedback: the previous shape remembered only the first unnumbered
candidate and fell through to a generic error if os.Open failed on it,
even though other matching unnumbered files in imgFiles could have
succeeded. The pre-PR code was more resilient because it looped and
continued on open failure.

fromExternalFile now collects every viable unnumbered candidate into a
slice during the scan, then tries them in order after the loop, mirroring
the pre-PR retry-on-open-failure behavior. Numbered matches still return
immediately on first success and skip the candidate list entirely — an
open failure on a numbered match means no other file has that number
anyway.

Also:
- globMetaChars doc comment now notes that '\' escape is intentionally
  excluded (filepath.Match supports it but treating it as a metachar here
  would misalign extractDiscNumber's literal-prefix extraction with no
  benefit for realistic config patterns).
- The 'cover.jpg doesn't match disc*.*' Entry in the extractDiscNumber
  table is renamed to 'cover.jpg with disc*.* (no prefix match)' to
  reflect that the test now exercises the HasPrefix defensive guard,
  not the removed internal filepath.Match check.

Regression test added: a single-folder album with a deleted first
candidate file resolves to the second candidate.

core/artwork/reader_disc.go

@@ -168,7 +168,11 @@ func (d *discArtworkReader) fromDiscSubtitle(ctx context.Context, subtitle strin
 	}
 }
 
-// globMetaChars lists every metacharacter understood by filepath.Match.
+// globMetaChars holds the substitution metacharacters understood by
+// filepath.Match. The '\' escape character is intentionally excluded:
+// disc art patterns come from user config and never include escaped
+// metachars in practice, and treating '\' as a metachar would misalign
+// the literal-prefix extraction in extractDiscNumber.
 const globMetaChars = "*?["
 
 // extractDiscNumber parses the disc number from a filename matched by a
@@ -208,7 +212,7 @@ func extractDiscNumber(pattern, filename string) (int, bool) {
 func (d *discArtworkReader) fromExternalFile(ctx context.Context, pattern string) sourceFunc {
 	isLiteral := !strings.ContainsAny(pattern, globMetaChars)
 	return func() (io.ReadCloser, string, error) {
-		var fallback string
+		var fallbacks []string
 		for _, file := range d.imgFiles {
 			_, name := filepath.Split(file)
 			name = strings.ToLower(name)
@@ -235,13 +239,10 @@ func (d *discArtworkReader) fromExternalFile(ctx context.Context, pattern string
 				}
 			}
 
-			if fallback != "" {
-				continue
-			}
 			if d.isMultiFolder && !d.discFolders[filepath.Dir(file)] {
 				continue
 			}
-			fallback = file
+			fallbacks = append(fallbacks, file)
 			// A literal filename can only match one file, so stop as soon
 			// as we have a viable fallback.
 			if isLiteral {
@@ -249,13 +250,13 @@ func (d *discArtworkReader) fromExternalFile(ctx context.Context, pattern string
 			}
 		}
 
-		if fallback != "" {
-			f, err := os.Open(fallback)
+		for _, file := range fallbacks {
+			f, err := os.Open(file)
 			if err != nil {
-				log.Warn(ctx, "Could not open disc art file", "file", fallback, err)
-			} else {
-				return f, fallback, nil
+				log.Warn(ctx, "Could not open disc art file", "file", file, err)
+				continue
 			}
+			return f, file, nil
 		}
 		return nil, "", fmt.Errorf("disc %d: pattern '%s' not matched by files", d.discNumber, pattern)
 	}

core/artwork/reader_disc_test.go

@@ -42,8 +42,8 @@ var _ = Describe("Disc Artwork Reader", func() {
 			// Case insensitive (filename already lowered by caller)
 			Entry("Disc1.jpg lowered", "disc*.*", "disc1.jpg", 1, true),
 
-			// Pattern doesn't match
-			Entry("cover.jpg doesn't match disc*.*", "disc*.*", "cover.jpg", 0, false),
+			// HasPrefix guard: filename doesn't share the pattern's literal prefix
+			Entry("cover.jpg with disc*.* (no prefix match)", "disc*.*", "cover.jpg", 0, false),
 
 			// Pattern with no wildcard before dot
 			Entry("front1.jpg with front*.*", "front*.*", "front1.jpg", 1, true),
@@ -207,6 +207,25 @@ var _ = Describe("Disc Artwork Reader", func() {
 			Entry("disc 3 falls back to disc.jpg when no numbered match exists", 3, 0),
 		)
 
+		It("tries the next fallback candidate when the first one cannot be opened", func() {
+			f1 := createFile("album/cover.jpg")
+			f2 := createFile("album/cover.png")
+			// Remove f1 so os.Open will fail on it; f2 should still win.
+			Expect(os.Remove(f1)).To(Succeed())
+			reader := &discArtworkReader{
+				discNumber:  1,
+				imgFiles:    []string{f1, f2},
+				discFolders: map[string]bool{filepath.Join(tmpDir, "album"): true},
+			}
+
+			sf := reader.fromExternalFile(ctx, "cover.*")
+			r, path, err := sf()
+			Expect(err).ToNot(HaveOccurred())
+			Expect(r).ToNot(BeNil())
+			r.Close()
+			Expect(path).To(Equal(f2))
+		})
+
 		DescribeTable("filters by disc number for non-'*' wildcard patterns",
 			func(pattern string, discNumber, expectedIdx int) {
 				files := []string{