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/log/log.go

@@ -266,7 +266,8 @@ func Progressf(format string, args ...interface{}) {
 	pterm.Printf("\r"+format, args...)
 }
 
-// ProgressDone completes a progress line by printing a newline.
+// ProgressDone clears the progress line when progress is complete.
 func ProgressDone() {
-	pterm.Println()
+	// Clear the line with spaces and return to beginning
+	pterm.Print("\r\033[K")
 }