An "ancient" range a group: articles 1 … low watermark − 1, never filled

Every group is tiled from article 1 now: one range with status 'ancient'
covers what had expired on the server before the group was planned.
Nothing selects it (leases, prefetcher, corpus fill, tail merger all take
'planned'), the status panel reports it apart, the search's history stats
leave it out. Migration 6700 adds it for the groups planned before;
Planner.historyRows (now a pure function, unit-tested) for new groups.
The status bands are ordered by an explicit key: the ancient row at
priority 0 tied "history 0-999" with "window 0".

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

REBUILD.md

@@ -60,6 +60,13 @@ most workers on batcave, a few on poweredge-kantoor with
 `edge:node --max-mbit` (e.g. 50 of its 160 Mbit/s ≈ 100–125 k articles/s;
 the cap is shared by the node's processes).
 
+Every group is tiled from article 1 (2026-10-28): one "ancient" range,
+status `ancient`, covers 1 … low watermark − 1, what had expired on the
+server before the group was planned. Never leased, fetched or filled —
+nothing selects it — and the status panel reports it apart ("Ancient: …
+articles below the low watermark"); migration 6700 added it for the
+groups planned before, `Planner.historyRows` for groups planned since.
+
 Also built (2026-10-09): the live tail (new articles planned every 2
 minutes from one LIST ACTIVE per name prefix, at priority −1: leased and
 fetched before history; since 2026-10-27 a round grows the group's open