Record each file's size on disk and print it in trees (closes #147)

The files table gets a disk_size column: the file's allocated blocks,
st_blocks × 512, from the lstat the walk already does. scan writes it
with every record it writes. When a rescan finds a file with its size
and mtime unchanged but a different disk_size, it updates disk_size
alone, without reading the file. trees adds disk_size up per directory
and prints it as a column after size. On a compressed or sparse file it
is less than size, so two copies of one tree can differ.

Judgement calls: each row's disk_size is the dupe copy's, not the
first's. A file whose record gets only a new disk_size counts as
unchanged.

Model: opus-5-5

TODO.md

@@ -29,6 +29,13 @@
 
 # Completed Steps
 
+- each record holds `disk_size`, the bytes the file takes on disk: its allocated
+  blocks, `st_blocks` × 512, from the lstat the walk already does. `trees`
+  prints a `disk_size` column after `size`, the sum over the `dupe` copy's
+  files; on a compressed or sparse file it differs from `size`. A rescan updates
+  the `disk_size` of an unchanged file whose size on disk changed, without
+  reading the file (2026-10-27, https://git.eeqj.de/sneak/sfdupes/issues/147)
+
 - the database keeps the number of records and the sum of their sizes in the one
   row of a `totals` table, which three triggers on `files` keep current. `info`
   reads that row, and so do `scan`'s `load` phase and the `read` phases of