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

TODO.md

@@ -29,10 +29,17 @@
 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. Its control characters are written out, as
+  `\t` or `\x1b`, so that the terminal does not act on them. The line is erased
+  when the walk ends, and the next phase is drawn on it (2026-11-11,
+  https://git.eeqj.de/sneak/sfdupes/issues/160)
+
 - `report` and `trees` leave out empty files. `report` writes no group of size
   0, and `trees` writes no group whose trees hold 0 bytes. An empty file still
   counts toward a tree's digest (2026-11-11,
   https://git.eeqj.de/sneak/sfdupes/issues/178)
+
 - 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