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>

start/env.ts

@@ -60,6 +60,7 @@ export default await Env.create(new URL('../', import.meta.url), {
   CHUNK_ARTICLES: Env.schema.number.optional(),
   LEASE_TTL_SECONDS: Env.schema.number.optional(),
   SPOOL_DIR: Env.schema.string.optional(),
+  CORPUS_DIR: Env.schema.string.optional(),
   SPOOL_AHEAD_PER_LEASE: Env.schema.number.optional(),
   SPOOL_MIN_AHEAD: Env.schema.number.optional(),
   SPOOL_MAX_GB: Env.schema.number.optional(),