Curation v2 step 6: Behind the paper, stats, run report block, lock

Behind-the-paper chapter (behind.xhtml, both editions) built from the run's
StageCounts and candidate_runs near misses; daily-epub stats [--days N];
StageCounts gains knn/feed gates and verdicts_in_prompt, timings split
into summaries + brief, the four-line §15.4 info block logged once per
run and printed by print_report; src/lock.rs flock guard on
<database_path>.lock for generate, profile rebuild, features backfill and
backfill-social; README updated for the new CLI, env vars and costs.

Implemented by a Claude agent from docs/plans/curation-v2-briefs/step6.md;
reviewed against plan §5, §15.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01A1rCLQeKBgnBo3oTgHuTMe

examples/image_audit.rs

@@ -417,6 +417,7 @@ fn write_audit_epub(out_dir: &Path, picks: &[Pick], assets: &[ImageAsset]) {
         },
         world_briefing: None,
         colophon: Colophon::default(),
+        behind: Default::default(),
     };
     let cfg = daily_epub::config::Config::default();
     match daily_epub::epub::build_edition_with_images(