REBUILD.md: the 2016 and 2021 windows marked for the corpus

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

REBUILD.md

@@ -256,7 +256,9 @@ group's ranges posted in the window `capture`: planned ones not started at
 fill, which takes capture ranges in priority order, done ones too (they're
 fetched again, corpus only), and skips those complete (`ranges.stored_at`,
 set when it sees the range.json). A year of every group is the data to
-replay voyager 3's 30-day blocks on.
+replay voyager 3's 30-day blocks on: marked 2026-10-25, 2016 (priority 0,
+30B articles, ~0.56 TB) and 2021 (priority 1, 87B, ~1.6 TB); a recent year
+is ~200B articles, ~3.6 TB.
 
 ## Phase 2