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>

schemas/delivery.capnp

@@ -20,9 +20,9 @@ interface DeliveryService {
   # recipientKey : Ed25519 public key of the intended recipient (exactly 32 bytes).
   # payload      : Opaque byte string — a TLS-encoded MlsMessageOut blob or any
   #                other framed data the application layer wants to deliver.
-  # channelId    : Optional channel identifier (empty for legacy). A 16-byte UUID
+  # channelId    : Optional channel identifier (empty for default). A 16-byte UUID
   #                is recommended for 1:1 channels.
-  # version      : Schema/wire version. Must be 0 (legacy) or 1 (this spec).
+  # version      : Schema/wire version. Must be 1.
   #
   # The payload is appended to the recipient's FIFO queue. Returns immediately;
   # the recipient retrieves it via `fetch`.
@@ -31,8 +31,8 @@ interface DeliveryService {
   # Fetch and atomically drain all queued payloads for a given recipient.
   #
   # recipientKey : Ed25519 public key of the caller (exactly 32 bytes).
-  # channelId    : Optional channel identifier (empty for legacy).
-  # version      : Schema/wire version. Must be 0 (legacy) or 1 (this spec).
+  # channelId    : Optional channel identifier (empty for default).
+  # version      : Schema/wire version. Must be 1.
   #
   # Returns the complete queue in FIFO order and clears it. Returns an empty
   # list if there are no pending messages.