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. 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

TODO.md

@@ -29,6 +29,11 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- On a terminal, the line below the `walk` display shows the path of the last
+  file the walk found, at each draw. The line is erased when the walk ends, and
+  the next phase is drawn on it (2026-11-10,
+  https://git.eeqj.de/sneak/sfdupes/issues/160)
+
 - The Go code moved out of the repository root. `cmd/sfdupes/main.go` is the
   command, and it calls `Main` in the package `internal/sfdupes`. `main.go`
   became `internal/sfdupes/cli.go`. Every other file kept its name. The install