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

The total summed `size` over every row, so a `dupe` inside another
row's `dupe` was counted twice. It now leaves out a row whose `dupe`
lies inside another row's `dupe`. 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, the bytes of
+  such a row were counted twice (2026-11-11,
+  https://git.eeqj.de/sneak/sfdupes/issues/168)
 - New `scan` flag `--exclude PATH`, which may be repeated. The walk neither
   enters nor records an excluded path, and keeps the records below it. An
   operand at or below one is a fatal error (2026-11-11,