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

The total summed `size` over every row, so a dupe tree inside another
reported dupe tree was counted twice. It is now what removing every
`dupe` path frees: a dupe tree with a dupe tree above it adds nothing.
The rows are unchanged. README defines the total to match.

Model: opus-5-5

TODO.md

@@ -29,6 +29,9 @@
 Newest first. An entry ends with its date and its issue or branch, where those
 are known.
 
+- The reclaimable total of `trees` counts each file once. A `dupe` tree inside
+  another `dupe` tree adds nothing, so the total is what removing every `dupe`
+  path frees (2026-11-11, https://git.eeqj.de/sneak/sfdupes/issues/168)
 - `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,