Merger: what's new an hour at a time, only the sets two releases share; common files match nothing

A round behind by two days of the probe backlog read 3.5M sets and never ended;
a par2 file shared by up to 190k files loaded every release it matched and the
merger ran out of heap every few minutes (LOG.md 2026-10-20 (85)).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

REBUILD.md

@@ -238,6 +238,10 @@ files the set describes, into the one with the most files (chunks,
 candidates, inspections, counts; the par2's shared name when none was
 readable); `release_merges` keeps what went where. Obfuscated uploads whose
 files were each a release of their own (`lone` candidates) become one.
+It reads what's new an hour (`created_at`) at a time, its place kept after
+each, and merges only the sets held or matched by two releases or more; a
+file whose first 16 KiB and size more than 20 files share is a common one and
+matches nothing (LOG.md 2026-10-20 (85)).
 
 **Backlog: voyager 1's features not in voyager 2** (CONCEPT.md decisions;
 voyager 1's data exported to `/mnt/bulk/usenet-rebuild/v1-export/` on