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/cli/fetch.go

@@ -13,6 +13,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/dustin/go-humanize"
 	"github.com/multiformats/go-multihash"
 	"github.com/urfave/cli/v2"
 	"sneak.berlin/go/mfer/internal/log"
@@ -89,12 +90,12 @@ func (mfa *CLIApp) fetchManifestOperation(ctx *cli.Context) error {
 		for p := range progress {
 			rate := formatBitrate(p.BytesPerSec * 8)
 			if p.ETA > 0 {
-				log.Infof("%s: %d/%d bytes, %s, ETA %s",
-					p.Path, p.BytesRead, p.TotalBytes,
+				log.Infof("%s: %s/%s, %s, ETA %s",
+					p.Path, humanize.IBytes(uint64(p.BytesRead)), humanize.IBytes(uint64(p.TotalBytes)),
 					rate, p.ETA.Round(time.Second))
 			} else {
-				log.Infof("%s: %d/%d bytes, %s",
-					p.Path, p.BytesRead, p.TotalBytes, rate)
+				log.Infof("%s: %s/%s, %s",
+					p.Path, humanize.IBytes(uint64(p.BytesRead)), humanize.IBytes(uint64(p.TotalBytes)), rate)
 			}
 		}
 	}()
@@ -129,9 +130,9 @@ func (mfa *CLIApp) fetchManifestOperation(ctx *cli.Context) error {
 	elapsed := time.Since(startTime)
 	avgBytesPerSec := float64(totalBytes) / elapsed.Seconds()
 	avgRate := formatBitrate(avgBytesPerSec * 8)
-	log.Infof("downloaded %d files (%.1f MB) in %.1fs (%s avg)",
+	log.Infof("downloaded %d files (%s) in %.1fs (%s avg)",
 		len(files),
-		float64(totalBytes)/1e6,
+		humanize.IBytes(uint64(totalBytes)),
 		elapsed.Seconds(),
 		avgRate)