fix: rating write — avoid rename, overwrite file in-place

bogem/id3v2 Save() renames a temp file over the original, which Windows
denies when Winamp holds the file open (no FILE_SHARE_DELETE). Fix:
- Use tag.WriteTo(&buf) to encode tag+audio into memory while the read
  handle is still open
- Close the read handle
- Reopen the original file with O_WRONLY|O_TRUNC and write the buffer

Winamp opens MP3s with FILE_SHARE_WRITE so the in-place overwrite succeeds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/rating/rating.go

@@ -13,8 +13,10 @@
 package rating
 
 import (
+	"bytes"
 	"fmt"
 	"math/big"
+	"os"
 	"path/filepath"
 	"strings"
 
@@ -47,6 +49,13 @@ func Get(path string) (int, error) {
 
 // Set writes a 0–5 star rating into the POPM frame of path.
 // stars=0 removes any existing POPM frame (unrated).
+//
+// bogem/id3v2's Save() writes a temp file and renames it over the original,
+// which Windows denies when another process (Winamp) holds the file open
+// without FILE_SHARE_DELETE. Instead we use WriteTo to stream the modified
+// tag+audio into an in-memory buffer, close our read handle, then overwrite
+// the original file in-place — which succeeds because Winamp opens files
+// with FILE_SHARE_WRITE.
 func Set(path string, stars int) error {
 	if !isMP3(path) {
 		return fmt.Errorf("rating.Set: not an MP3 file: %s", filepath.Base(path))
@@ -57,9 +66,8 @@ func Set(path string, stars int) error {
 
 	tag, err := id3.Open(path, id3.Options{Parse: true})
 	if err != nil {
-		return fmt.Errorf("rating.Set: %w", err)
+		return fmt.Errorf("rating.Set: open: %w", err)
 	}
-	defer tag.Close()
 
 	tag.DeleteFrames("POPM")
 	if stars > 0 {
@@ -69,8 +77,23 @@ func Set(path string, stars int) error {
 			Counter: big.NewInt(0),
 		})
 	}
-	if err := tag.Save(); err != nil {
-		return fmt.Errorf("rating.Set: save %s: %w", filepath.Base(path), err)
+
+	// Stream modified tag + audio into memory buffer while file is still open.
+	var buf bytes.Buffer
+	if _, err := tag.WriteTo(&buf); err != nil {
+		tag.Close()
+		return fmt.Errorf("rating.Set: encode: %w", err)
+	}
+	tag.Close() // release read handle before we open for writing
+
+	// Overwrite the original file in-place (no rename needed).
+	f, err := os.OpenFile(path, os.O_WRONLY|os.O_TRUNC, 0644)
+	if err != nil {
+		return fmt.Errorf("rating.Set: open for write: %w", err)
+	}
+	defer f.Close()
+	if _, err := f.Write(buf.Bytes()); err != nil {
+		return fmt.Errorf("rating.Set: write: %w", err)
 	}
 	return nil
 }