Show the walk's path on the line below its display (closes #160)

On a terminal, each draw of the walk shows the path of the last file
it found on the line below, cut short before the last column with
wide characters counted as two columns. The progressbar library's
detail row draws it. Control characters in the path are written out,
as \t or \x1b, so that the terminal does not act on them. The line is
erased before a warning and when the walk ends, so the next phase is
drawn on it. Nothing changes when stderr is not a terminal.

github.com/rivo/uniseg, already in the module graph through the
progressbar library, becomes a direct dependency for the column count.

Model: opus-5-5

README.md

@@ -1201,6 +1201,14 @@ On a terminal:
     ⠹ [3/4] hash: [12345/98765], 92 files/s, elapsed 2m32s, remain 17m44s            12% |█         |
     ```
 
+- During `scan`'s `walk`, the line below the display shows the path of the last
+  file the walk found. Each draw shows the path of that moment, cut short before
+  the terminal's last column. The path is escaped as in the reports (see "Report
+  output format"). Each control character left after that is written as `\x` and
+  its code in two hexadecimal digits, such as `\x1b` for the escape character. A
+  control character is one that a terminal acts on instead of showing. The line
+  is erased when the walk ends, and the next phase is drawn on it.
+
 - No line reaches the terminal's last column. A line too wide for the terminal
   first leaves out its rate, the words of a stalled phase, or what the phase is
   waiting for. It leaves out its bar next. It is cut short at the edge only when