trees: count a nested dupe tree once in the reclaimable total (closes #168)

The total summed `size` over every row. It now leaves out a row whose
`dupe` lies inside another row's `dupe`. Before, such a row's bytes were
counted again in each row whose `dupe` holds it. The rows are unchanged.
README defines the total to match.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- The reclaimable total of `trees` is the sum of `size` over all rows, leaving
+  out a row whose `dupe` lies inside another row's `dupe`. Before, such a row's
+  bytes were counted again in each row whose `dupe` holds it (2026-11-11,
+  https://git.eeqj.de/sneak/sfdupes/issues/168)
 - `dwim` takes the `scan` flags `--workers`, `-x`, `--no-default-excludes` and
   `--exclude`, with the same meaning. Both commands register and check them with
   the same code (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/162)