Stream report and trees instead of loading every record (closes #14)

report now has SQLite group the records and put the rows in report
order, helped by a new files_signature index on (size, head, tail,
content), and writes each row as it reads it. trees reads the records
in path order, where all the paths under a directory come together, so
it computes each directory's digest as soon as the stream leaves it and
keeps only its path, parent, digest and totals. Output is unchanged.

The tests that called the removed in-memory grouping functions now group
records stored in a database. A new test checks that both commands give
the same output whatever order the records were inserted in.

Model: opus-5-5

TODO.md

@@ -29,6 +29,10 @@
 
 # Completed Steps
 
+- `report` and `trees` stream the records instead of holding them all in
+  memory; the schema gains the `files_signature` index (2026-10-04,
+  https://git.eeqj.de/sneak/sfdupes/issues/14)
+
 - `scan` holds a lock on a lock file beside the database for its whole run,
   so a second `scan` fails at once with exit 1 (2026-10-03,
   https://git.eeqj.de/sneak/sfdupes/issues/53)