feat: Phase 9 — developer experience, extensibility, and community growth

New crates:
- quicproquo-bot: Bot SDK with polling API + JSON pipe mode
- quicproquo-kt: Key Transparency Merkle log (RFC 9162 subset)
- quicproquo-plugin-api: no_std C-compatible plugin vtable API
- quicproquo-gen: scaffolding tool (qpq-gen plugin/bot/rpc/hook)

Server features:
- ServerHooks trait wired into all RPC handlers (enqueue, fetch, auth,
  channel, registration) with plugin rejection support
- Dynamic plugin loader (libloading) with --plugin-dir config
- Delivery proof canary tokens (Ed25519 server signatures on enqueue)
- Key Transparency Merkle log with inclusion proofs on resolveUser

Core library:
- Safety numbers (60-digit HMAC-SHA256 key verification codes)
- Verifiable transcript archive (CBOR + ChaCha20-Poly1305 + hash chain)
- Delivery proof verification utility
- Criterion benchmarks (hybrid KEM, MLS, identity, sealed sender, padding)

Client:
- /verify REPL command for out-of-band key verification
- Full-screen TUI via Ratatui (feature-gated --features tui)
- qpq export / qpq export-verify CLI subcommands
- KT inclusion proof verification on user resolution

Also: ROADMAP Phase 9 added, bot SDK docs, server hooks docs,
crate-responsibilities updated, example plugins (rate_limit, logging).

crates/quicproquo-client/src/client/repl.rs

@@ -11,7 +11,7 @@ use std::time::Duration;
 use anyhow::Context;
 use quicproquo_core::{
     AppMessage, DiskKeyStore, GroupMember, IdentityKeypair, ReceivedMessage,
-    hybrid_encrypt, parse as parse_app_msg, serialize_chat,
+    compute_safety_number, hybrid_encrypt, parse as parse_app_msg, serialize_chat,
 };
 use quicproquo_proto::node_capnp::node_service;
 use tokio::sync::mpsc;
@@ -57,6 +57,8 @@ enum SlashCommand {
     /// Mesh subcommands: /mesh peers, /mesh server <addr>
     MeshPeers,
     MeshServer { addr: String },
+    /// Display safety number for out-of-band key verification with a contact.
+    Verify { username: String },
 }
 
 fn parse_input(line: &str) -> Input {
@@ -135,6 +137,13 @@ fn parse_input(line: &str) -> Input {
                 Input::Empty
             }
         },
+        "/verify" => match arg {
+            Some(username) => Input::Slash(SlashCommand::Verify { username }),
+            None => {
+                display::print_error("usage: /verify <username>");
+                Input::Empty
+            }
+        },
         _ => {
             display::print_error(&format!("unknown command: {cmd}. Try /help"));
             Input::Empty
@@ -601,6 +610,7 @@ async fn handle_slash(
             ));
             Ok(())
         }
+        SlashCommand::Verify { username } => cmd_verify(session, client, &username).await,
     };
     if let Err(e) = result {
         display::print_error(&format!("{e:#}"));
@@ -622,6 +632,7 @@ fn print_help() {
     display::print_status("  /whoami                     - Show your identity");
     display::print_status("  /mesh peers                 - Discover nearby qpq nodes via mDNS");
     display::print_status("  /mesh server <host:port>    - Show how to reconnect to a mesh node");
+    display::print_status("  /verify <username>          - Show safety number for key verification");
     display::print_status("  /quit                       - Exit");
 }
 
@@ -1200,6 +1211,43 @@ fn cmd_history(session: &SessionState, count: usize) -> anyhow::Result<()> {
     Ok(())
 }
 
+async fn cmd_verify(
+    session: &SessionState,
+    client: &node_service::Client,
+    username: &str,
+) -> anyhow::Result<()> {
+    // Resolve contact's identity key from the server.
+    display::print_status(&format!("resolving {username}..."));
+    let peer_key_vec = resolve_user(client, username)
+        .await?
+        .with_context(|| format!("user '{username}' not found"))?;
+
+    anyhow::ensure!(
+        peer_key_vec.len() == 32,
+        "server returned an identity key with unexpected length ({}); expected 32 bytes",
+        peer_key_vec.len()
+    );
+
+    let peer_key: [u8; 32] = peer_key_vec
+        .as_slice()
+        .try_into()
+        .expect("length checked above");
+
+    let my_key: [u8; 32] = session.identity.public_key_bytes();
+
+    let safety_number = compute_safety_number(&my_key, &peer_key);
+
+    display::print_status(&format!("Safety number with @{username}:"));
+    display::print_status("");
+    display::print_status(&format!("  {safety_number}"));
+    display::print_status("");
+    display::print_status("Compare this number with your contact via a separate channel");
+    display::print_status("(voice call, in person, or any out-of-band means).");
+    display::print_status("If the numbers match, the connection has not been tampered with.");
+
+    Ok(())
+}
+
 // ── Sending ──────────────────────────────────────────────────────────────────
 
 async fn handle_send(