feat: add protocol comparison docs, P2P crate, production audit, and design fixes

Add comprehensive documentation comparing quicnprotochat against classical
chat protocols (IRC+SSL, XMPP, Telegram) with diagrams and attack scenarios.
Promote comparison pages to top-level sidebar section. Include P2P transport
crate (iroh), production readiness audit, CI workflows, dependency policy,
and continued architecture improvements across all crates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -3,3 +3,8 @@
 .vscode/
 gitea-mcp.json
 docs/book/
+
+# Server/client runtime data — do not commit certs, keys, or DBs
+data/
+*.der
+quicnprotochat-server.toml