fix: ensure output dirs exist in extract and API extract

cmdExtract and handleAPIExtract were missing os.MkdirAll calls
while cmdDownload, cmdPipeline, and handleAPIDownload all had
them. The bash script creates dirs internally but this is not
robust — create dirs in Go before shelling out.

Closes #13

main.go

@@ -391,6 +391,8 @@ func cmdExtract(args []string) {
 		toolPath = findScript("extract_wavs.sh")
 	}
 
+	os.MkdirAll(*outDir, 0755)
+
 	entries, err := os.ReadDir(*isoDir)
 	if err != nil {
 		fatal("read dir: %v", err)

serve.go

@@ -287,6 +287,8 @@ func handleAPIExtract(w http.ResponseWriter, r *http.Request) {
 		outDir = "./wavs"
 	}
 
+	os.MkdirAll(outDir, 0755)
+
 	toolPath := findScript("extract_wavs.sh")
 
 	entries, err := os.ReadDir(isoDir)