Print the byte unit once in the summary line (closes #17)

formatBytes already ends in a unit, so the summary line read "9 B
bytes" or "1.0 KiB bytes". The format string in Stats.Print no longer
adds the word "bytes".

Print now takes the io.Writer to print to; its callers pass os.Stderr,
and the new test passes a buffer to read the line for one small and
one large byte count.

Model: opus-5-5

TODO.md

@@ -25,6 +25,8 @@ today), `.dockerignore`, `.gitea/workflows/check.yml` and
 
 # Completed Steps
 
+* 2026-10-06: the summary line after `sum`, `check` and `clear` prints
+    the byte unit once (`1.5 KiB`, not `1.5 KiB bytes`)
 * 2026-10-06: lint and test run as phases of the `Dockerfile`, and the
     build stage depends on both; `script/lint` and `script/test` each
     build their phase with `--no-cache`; `script/cibuild` bootstraps,