refactor: add helper wrappers for stdin/stdout/stderr IO

Address all four review concerns on PR #31:

1. Fix missed bare fmt.Println() in VerifySnapshotWithOptions (line 620)
2. Replace all direct fmt.Fprintf(v.Stdout,...) / fmt.Fprintln(v.Stdout,...) /
   fmt.Fscanln(v.Stdin,...) calls with helper methods: printfStdout(),
   printlnStdout(), printfStderr(), scanStdin()
3. Route progress bar and stderr output through v.Stderr instead of os.Stderr
   in restore.go (concern #4: v.Stderr now actually used)
4. Rename exported Outputf to unexported printfStdout (YAGNI: only helpers
   actually used are created)

internal/vaultik/vaultik.go

@@ -129,12 +129,6 @@ func (v *Vaultik) GetFilesystem() afero.Fs {
 	return v.Fs
 }
 
-// Outputf writes formatted output to stdout for user-facing messages.
-// This should be used for all non-log user output.
-func (v *Vaultik) Outputf(format string, args ...any) {
-	_, _ = fmt.Fprintf(v.Stdout, format, args...)
-}
-
 // printfStdout writes formatted output to stdout.
 func (v *Vaultik) printfStdout(format string, args ...any) {
 	_, _ = fmt.Fprintf(v.Stdout, format, args...)
@@ -145,10 +139,14 @@ func (v *Vaultik) printlnStdout(args ...any) {
 	_, _ = fmt.Fprintln(v.Stdout, args...)
 }
 
-// scanlnStdin reads a line from stdin into the provided string pointer.
-func (v *Vaultik) scanlnStdin(s *string) error {
-	_, err := fmt.Fscanln(v.Stdin, s)
-	return err
+// printfStderr writes formatted output to stderr.
+func (v *Vaultik) printfStderr(format string, args ...any) {
+	_, _ = fmt.Fprintf(v.Stderr, format, args...)
+}
+
+// scanStdin reads a line of input from stdin.
+func (v *Vaultik) scanStdin(a ...any) (int, error) {
+	return fmt.Fscanln(v.Stdin, a...)
 }
 
 // FetchBlob downloads a blob from storage and returns a reader for the encrypted data.
@@ -162,7 +160,6 @@ func (v *Vaultik) FetchBlob(ctx context.Context, blobHash string, expectedSize i
 
 	return reader, expectedSize, nil
 }
-
 // TestVaultik wraps a Vaultik with captured stdout/stderr for testing
 type TestVaultik struct {
 	*Vaultik