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>

docs/src/introduction.md

@@ -64,6 +64,7 @@ For a deeper discussion of the cryptographic guarantees, threat model, and known
 
 | Section | What you will find |
 |---|---|
+| **[Comparison with Classical Protocols](design-rationale/protocol-comparison.md)** | **Why quicnprotochat? IRC+SSL, XMPP, Telegram vs. our design** |
 | [Prerequisites](getting-started/prerequisites.md) | Toolchain and system dependencies |
 | [Building from Source](getting-started/building.md) | `cargo build`, Cap'n Proto codegen, troubleshooting |
 | [Running the Server](getting-started/running-the-server.md) | Server startup, configuration, TLS cert generation |
@@ -74,7 +75,7 @@ For a deeper discussion of the cryptographic guarantees, threat model, and known
 | [Protocol Layers](protocol-layers/overview.md) | Deep dives into QUIC/TLS, Cap'n Proto, MLS, Hybrid KEM |
 | [Wire Format Reference](wire-format/overview.md) | Cap'n Proto schema documentation |
 | [Cryptography](cryptography/overview.md) | Identity keys, key lifecycle, forward secrecy, PCS, threat model |
-| [Design Rationale](design-rationale/overview.md) | ADRs and "why not Signal/Matrix" comparison |
+| [Design Rationale](design-rationale/overview.md) | ADRs and protocol design decisions |
 | [Roadmap](roadmap/milestones.md) | Milestone tracker and future research directions |
 
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