Rebuild spec: d and e decided; coordinator prefetches ranges into a spool

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

REBUILD.md

@@ -18,7 +18,20 @@ dropping garbage; 28 TB of compressed overview traffic. Throughput goal:
 1. **Coordinator** (server). Plans each group's history as ranges of 2 M
    articles, leases them to workers, tracks progress and retries. The
    group list (owner rule) and the garbage signature table go with every
-   lease. Fetches XOVER for relay workers (gzip stream passed on as is).
+   lease. Fetches XOVER for the workers (relay: workers have no NNTP
+   account; the gzip stream is passed on as is).
+
+   **Prefetch**: the coordinator fetches the next ranges ahead of the
+   workers, in chunks of 50 k lines and in order, into a spool on disk
+   (~106 MB per 2 M range at ~53 B per line; on the HDD, written and read
+   sequentially). It keeps about two ranges ahead per active lease, capped
+   by size (default 10 GB), and fetches with all its header connections all
+   the time, independent of how fast workers are. A lease goes to a range
+   that is already (partly) spooled; a worker reads its chunks from the
+   spool, waiting only for chunks still being fetched, and falls back to a
+   direct relay fetch for a range that isn't spooled. Spooled chunks are
+   deleted when their range's results are stored, and fetched again if a
+   range is leased again later.
    Order: oldest era first (2008–2018 holds 63% of the readable files on
    17% of the lines), then forward.
 
@@ -107,6 +120,6 @@ a. Chunk payload: a write-once `bytea` table in Postgres, partitioned by
    month, on the HDD; files on disk only if writes still limit. **Accepted.**
 b. A separate database on the same server. **Accepted.**
 c. Group list: editable, owner by priority number (above). **Accepted.**
-d. Reposts (same release posted again): separate releases, or merged by
-   name and size in the catalog. Proposal: separate, merged later in phase 2.
-e. Headers through the relay (as now), or workers with their own accounts.
+d. Reposts: separate releases in phase 1, merged in phase 2. **Accepted.**
+e. Headers through the coordinator's relay, with prefetch (part 1).
+   **Accepted.**