Add atomic writes, humanized sizes, debug logging, and -v/-q per-command

- Atomic writes for mfer gen: writes to temp file, renames on success,
  cleans up temp on error/interrupt. Prevents empty manifests on Ctrl-C.
- Humanized byte sizes using dustin/go-humanize (e.g., "10 MiB" not "10485760")
- Progress lines clear when done (using ANSI escape \r\033[K])
- Debug logging when files are added to manifest (mfer gen -vv)
- Move -v/-q flags from global to per-command for better UX
- Add tests for atomic write behavior with failing filesystem mock

internal/scanner/scanner.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/spf13/afero"
+	"sneak.berlin/go/mfer/internal/log"
 	"sneak.berlin/go/mfer/mfer"
 )
 
@@ -353,6 +354,8 @@ func (s *Scanner) ToManifest(ctx context.Context, w io.Writer, progress chan<- S
 			return err
 		}
 
+		log.Debugf("+ %s (%d bytes)", entry.Path, bytesRead)
+
 		scannedFiles++
 		scannedBytes += bytesRead
 	}