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

Cargo.toml

@@ -18,6 +18,7 @@ hex = "0.4.3"
 hmac = "0.13.0"
 image = "0.25.10"
 jiff = { version = "0.2.35", features = ["serde"] }
+libc = "0.2.189"
 rand = "0.10.2"
 reqwest = { version = "0.13.4", default-features = false, features = ["rustls", "gzip", "json", "charset", "http2", "query", "system-proxy"] }
 resvg = "0.48.1"