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

src/curate/profile/mod.rs

@@ -304,6 +304,7 @@ pub async fn load_or_build(
         ),
         version,
         built_at,
+        verdicts: ratings.len().min(verdict_limit),
     };
     db.kv_set(KV_TASTE_PROFILE, &profile.text).await?;
     tracing::debug!(
@@ -472,6 +473,7 @@ pub async fn rebuild(
         ),
         version: next_version,
         built_at,
+        verdicts: current.len().min(verdict_limit),
     };
     db.kv_set(KV_TASTE_PROFILE, &profile.text).await?;
     tracing::info!(