feat(music): friendly cross-platform ffmpeg-missing error

internal/music/music.go

@@ -1,6 +1,7 @@
 package music
 
 import (
+	"fmt"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -104,7 +105,7 @@ func ResolveFFmpegWithConfig(configuredPath string) (string, error) {
 	}
 	path, err := exec.LookPath("ffmpeg")
 	if err != nil {
-		return "", err
+		return "", ffmpegMissingErr("ffmpeg")
 	}
 	return path, nil
 }
@@ -122,11 +123,23 @@ func ResolveFFprobeWithConfig(configuredPath string) (string, error) {
 	}
 	path, err := exec.LookPath("ffprobe")
 	if err != nil {
-		return "", err
+		return "", ffmpegMissingErr("ffprobe")
 	}
 	return path, nil
 }
 
+// ffmpegMissingErr renders an actionable, cross-platform "tool not found"
+// message. Only the music builder needs ffmpeg/ffprobe; every other builder
+// runs with zero external dependencies.
+func ffmpegMissingErr(tool string) error {
+	return fmt.Errorf(`%[1]s not found on PATH.
+  Windows: winget install Gyan.FFmpeg
+  macOS:   brew install ffmpeg
+  Linux:   apt install ffmpeg   (or your distro's package manager)
+Only the music builder needs %[1]s; set SOW_%[2]s to a full path to override.`,
+		tool, strings.ToUpper(tool))
+}
+
 func IsMusicAssetPath(rel string) bool {
 	rel = filepath.ToSlash(strings.TrimSpace(rel))
 	return rel == "envi/music" || strings.HasPrefix(rel, "envi/music/")

internal/music/music_test.go

@@ -402,3 +402,16 @@ func TestResolveFFprobeEnv(t *testing.T) {
 		t.Fatalf("expected '/custom/ffprobe', got %q", path)
 	}
 }
+
+func TestFFmpegMissingErrHasInstallHints(t *testing.T) {
+	msg := ffmpegMissingErr("ffmpeg").Error()
+	for _, want := range []string{"ffmpeg", "winget", "brew", "apt", "SOW_FFMPEG"} {
+		if !strings.Contains(msg, want) {
+			t.Errorf("ffmpeg error missing %q; got:\n%s", want, msg)
+		}
+	}
+	probe := ffmpegMissingErr("ffprobe").Error()
+	if !strings.Contains(probe, "SOW_FFPROBE") {
+		t.Errorf("ffprobe error should mention SOW_FFPROBE; got:\n%s", probe)
+	}
+}