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.

internal/checker/checker.go

@@ -8,6 +8,7 @@ import (
 	"io"
 	"os"
 	"path/filepath"
+	"time"
 
 	"github.com/multiformats/go-multihash"
 	"github.com/spf13/afero"
@@ -54,12 +55,13 @@ func (s Status) String() string {
 
 // CheckStatus contains progress information for the check operation.
 type CheckStatus struct {
-	TotalFiles   int64   // Total number of files in manifest
-	CheckedFiles int64   // Number of files checked so far
-	TotalBytes   int64   // Total bytes to verify (sum of all file sizes)
-	CheckedBytes int64   // Bytes verified so far
-	BytesPerSec  float64 // Current throughput rate
-	Failures     int64   // Number of verification failures encountered
+	TotalFiles   int64         // Total number of files in manifest
+	CheckedFiles int64         // Number of files checked so far
+	TotalBytes   int64         // Total bytes to verify (sum of all file sizes)
+	CheckedBytes int64         // Bytes verified so far
+	BytesPerSec  float64       // Current throughput rate
+	ETA          time.Duration // Estimated time to completion
+	Failures     int64         // Number of verification failures encountered
 }
 
 // Checker verifies files against a manifest.
@@ -136,6 +138,8 @@ func (c *Checker) Check(ctx context.Context, results chan<- Result, progress cha
 	var checkedBytes int64
 	var failures int64
 
+	startTime := time.Now()
+
 	for _, entry := range c.files {
 		select {
 		case <-ctx.Done():
@@ -153,13 +157,27 @@ func (c *Checker) Check(ctx context.Context, results chan<- Result, progress cha
 			results <- result
 		}
 
-		// Send progress (simplified - every file for now)
+		// Send progress with rate and ETA calculation
 		if progress != nil {
+			elapsed := time.Since(startTime)
+			var bytesPerSec float64
+			var eta time.Duration
+
+			if elapsed > 0 && checkedBytes > 0 {
+				bytesPerSec = float64(checkedBytes) / elapsed.Seconds()
+				remainingBytes := totalBytes - checkedBytes
+				if bytesPerSec > 0 {
+					eta = time.Duration(float64(remainingBytes)/bytesPerSec) * time.Second
+				}
+			}
+
 			sendCheckStatus(progress, CheckStatus{
 				TotalFiles:   totalFiles,
 				CheckedFiles: checkedFiles,
 				TotalBytes:   totalBytes,
 				CheckedBytes: checkedBytes,
+				BytesPerSec:  bytesPerSec,
+				ETA:          eta,
 				Failures:     failures,
 			})
 		}