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

progress.go

@@ -139,6 +139,13 @@ func (p *progress) warnf(format string, args ...any) {
 	}
 
 	fmt.Fprintln(os.Stderr, msg)
+
+	// The library never draws a finished bar again, so the warning for
+	// a pass's last item would erase it for good: draw its last frame
+	// again below the warning.
+	if p.bar != nil && p.bar.IsFinished() {
+		fmt.Fprint(os.Stderr, p.bar.String())
+	}
 }
 
 // finish terminates the pass's display. A bar whose pass stopped short