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>

config/coordinator.ts

@@ -75,6 +75,10 @@ export default {
     /** Candidates claimed at a time. */
     batch: 200,
   },
+  /** Stored XOVER chunks of ranges marked `capture`, for the grouping sweeps (app/services/corpus.ts). Off when unset. */
+  corpus: {
+    dir: env.get('CORPUS_DIR'),
+  },
   spool: {
     dir: env.get('SPOOL_DIR', app.tmpPath('spool')),
     /** Ranges kept fetched ahead, per active lease. */