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

README.md

@@ -802,7 +802,8 @@ The summary line on stderr gives:
 - the records read;
 - the duplicate-tree groups;
 - the dupe trees, which is the number of rows;
-- the reclaimable bytes, the sum of `size` over all rows.
+- the reclaimable bytes, what removing every `dupe` path would free. A `dupe`
+  path inside another `dupe` path adds nothing, so each file is counted once.
 
 ```
 trees: 14 records read, 1 duplicate tree groups, 1 dupe trees, 3.0 KiB reclaimable