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>

edge/src/indexer.rs

@@ -47,8 +47,12 @@ pub struct ReleaseChunk {
     pub readable: bool,
     pub has_par2: bool,
     pub payload: Vec<u8>,
+    /// The first files' subjects (at most SUBJECTS): edge-bench's --releases dump for the grouping sweeps.
+    pub subjects: Vec<String>,
 }
 
+pub const SUBJECTS: usize = 3;
+
 pub struct Candidate {
     pub release_key: String,
     pub upload: usize,
@@ -830,6 +834,7 @@ fn chunk_of(key: &str, files: &[Vec<Seg>], upload: usize) -> ReleaseChunk {
         readable: chunk_files.iter().any(|f| !is_random_file(f.name)),
         has_par2: chunk_files.iter().any(|f| names::is_par2(f.name)),
         payload: encode_chunk(&chunk_files),
+        subjects: chunk_files.iter().take(SUBJECTS).map(|f| f.subject.to_string()).collect(),
     }
 }