Redraw a finished bar below a warning for its last file (closes #102)

On a terminal, warnf clears the bar's line before it prints a warning.
When the warning is for the last item of a phase, the bar has already
drawn its finished frame and the progress library draws nothing more
for a finished bar, so the warning took the bar's place for good. warnf
now draws the finished bar's last frame again below the warning.
Output when stderr is not a terminal does not change.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- on a terminal, a warning for the last file of a phase no longer erases the
+  phase's finished bar: the bar is drawn again below the warning (2026-10-14,
+  https://git.eeqj.de/sneak/sfdupes/issues/102)
+
 - `scan` records mtime to the nanosecond, as whole seconds in `mtime` plus
   `mtime_nsec`, and compares it at that resolution, so a same-size rewrite
   within the same second is re-hashed (2026-10-07,