Corpus: stored XOVER chunks for the grouping sweeps

Ranges marked `capture` keep their XOVER chunks in CORPUS_DIR
(corpus/<group>/<start>-<end>/, edge-bench's layout, range.json once
complete; only chunks that inflate). The prefetcher reads a stored chunk
instead of fetching it, so a reset re-indexes them without the news
server; a chunk a worker reports corrupt goes from the corpus too.

edge-bench gets --releases (release chunks without payload, with their
first subjects) and ships in the edge image; deploy/corpus_sweep.py marks
more ranges (capture), replays the corpus with the deployed build (replay)
and reports suspects and the change since the previous build (report).

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

deploy/compose.yaml

@@ -19,8 +19,11 @@ x-app: &app
     DB_PORT: 5432
     APP_URL: http://web:3334
     SPOOL_DIR: /spool
+    # Stored XOVER chunks of the ranges marked `capture` (the grouping sweeps' corpus, HDD; app/services/corpus.ts).
+    CORPUS_DIR: /corpus
   volumes:
     - ./spool:/spool
+    - ./corpus:/corpus
   stop_grace_period: 60s
   depends_on:
     migrate: