Change FileProgress callback to channel-based progress

Replace callback-based progress reporting in Builder.AddFile with
channel-based FileHashProgress for consistency with EnumerateStatus
and ScanStatus patterns. Update scanner.go to use the new channel API.

mfer/builder.go

@@ -9,8 +9,10 @@ import (
 	"github.com/multiformats/go-multihash"
 )
 
-// FileProgress is called during file processing to report bytes read.
-type FileProgress func(bytesRead int64)
+// FileHashProgress reports progress during file hashing.
+type FileHashProgress struct {
+	BytesRead int64 // Total bytes read so far for the current file
+}
 
 // Builder constructs a manifest by adding files one at a time.
 type Builder struct {
@@ -28,14 +30,14 @@ func NewBuilder() *Builder {
 }
 
 // AddFile reads file content from reader, computes hashes, and adds to manifest.
-// The progress callback is called periodically with total bytes read so far.
+// Progress updates are sent to the progress channel (if non-nil) without blocking.
 // Returns the number of bytes read.
 func (b *Builder) AddFile(
 	path string,
 	size int64,
 	mtime time.Time,
 	reader io.Reader,
-	progress FileProgress,
+	progress chan<- FileHashProgress,
 ) (int64, error) {
 	// Create hash writer
 	h := sha256.New()
@@ -49,9 +51,7 @@ func (b *Builder) AddFile(
 		if n > 0 {
 			h.Write(buf[:n])
 			totalRead += int64(n)
-			if progress != nil {
-				progress(totalRead)
-			}
+			sendFileHashProgress(progress, FileHashProgress{BytesRead: totalRead})
 		}
 		if err == io.EOF {
 			break
@@ -84,6 +84,17 @@ func (b *Builder) AddFile(
 	return totalRead, nil
 }
 
+// sendFileHashProgress sends a progress update without blocking.
+func sendFileHashProgress(ch chan<- FileHashProgress, p FileHashProgress) {
+	if ch == nil {
+		return
+	}
+	select {
+	case ch <- p:
+	default:
+	}
+}
+
 // FileCount returns the number of files added to the builder.
 func (b *Builder) FileCount() int {
 	b.mu.Lock()
@@ -91,6 +102,23 @@ func (b *Builder) FileCount() int {
 	return len(b.files)
 }
 
+// AddFileWithHash adds a file entry with a pre-computed hash.
+// This is useful when the hash is already known (e.g., from an existing manifest).
+func (b *Builder) AddFileWithHash(path string, size int64, mtime time.Time, hash []byte) {
+	entry := &MFFilePath{
+		Path: path,
+		Size: size,
+		Hashes: []*MFFileChecksum{
+			{MultiHash: hash},
+		},
+		Mtime: newTimestampFromTime(mtime),
+	}
+
+	b.mu.Lock()
+	b.files = append(b.files, entry)
+	b.mu.Unlock()
+}
+
 // Build finalizes the manifest and writes it to the writer.
 func (b *Builder) Build(w io.Writer) error {
 	b.mu.Lock()