feat: Sprint 2 — security hardening, MLS key rotation, E2E tests

- DS sender identity binding (Phase 4.3): explicit audit logging of
  sender_prefix in enqueue/batch_enqueue, documenting that sender
  identity is always derived from authenticated session
- Username enumeration mitigation (Phase 4.5): 5ms timing floor on
  resolveUser responses + rate limiting to prevent bulk enumeration
- Add /update-key REPL command for MLS leaf key rotation via
  propose_self_update + auto-commit + fan-out to group members
- Add 4 new E2E tests: message delivery round-trip, key rotation
  update path, oversized payload rejection, multi-party group (12 total)

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

@@ -59,6 +59,8 @@ enum SlashCommand {
     MeshServer { addr: String },
     /// Display safety number for out-of-band key verification with a contact.
     Verify { username: String },
+    /// Rotate own MLS leaf key in the active group.
+    UpdateKey,
 }
 
 fn parse_input(line: &str) -> Input {
@@ -144,6 +146,7 @@ fn parse_input(line: &str) -> Input {
                 Input::Empty
             }
         },
+        "/update-key" | "/rotate-key" => Input::Slash(SlashCommand::UpdateKey),
         _ => {
             display::print_error(&format!("unknown command: {cmd}. Try /help"));
             Input::Empty
@@ -613,6 +616,7 @@ async fn handle_slash(
             Ok(())
         }
         SlashCommand::Verify { username } => cmd_verify(session, client, &username).await,
+        SlashCommand::UpdateKey => cmd_update_key(session, client).await,
     };
     if let Err(e) = result {
         display::print_error(&format!("{e:#}"));
@@ -634,6 +638,7 @@ 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("  /update-key                 - Rotate your MLS leaf key in the active group");
     display::print_status("  /verify <username>          - Show safety number for key verification");
     display::print_status("  /quit                       - Exit");
 }
@@ -1076,6 +1081,51 @@ async fn cmd_leave(
     Ok(())
 }
 
+async fn cmd_update_key(
+    session: &mut SessionState,
+    client: &node_service::Client,
+) -> anyhow::Result<()> {
+    let conv_id = session
+        .active_conversation
+        .as_ref()
+        .context("no active conversation; switch to a group first")?
+        .clone();
+
+    let my_key = session.identity_bytes();
+
+    let member = session
+        .get_member_mut(&conv_id)
+        .context("no group member for active conversation")?;
+
+    anyhow::ensure!(
+        member.group_ref().is_some(),
+        "active conversation has no MLS group"
+    );
+
+    // Propose a self-update (leaf key rotation).
+    let proposal = member.propose_self_update()?;
+
+    // Immediately commit the pending proposal.
+    let (commit, _welcome) = member.commit_pending_proposals()?;
+
+    // Fan out the commit to all other group members.
+    let others: Vec<Vec<u8>> = member
+        .member_identities()
+        .into_iter()
+        .filter(|id| id.as_slice() != my_key.as_slice())
+        .collect();
+
+    // Send proposal followed by commit so recipients can process in order.
+    for rk in &others {
+        enqueue(client, rk, &proposal).await?;
+        enqueue(client, rk, &commit).await?;
+    }
+
+    session.save_member(&conv_id)?;
+    display::print_status("key rotation complete");
+    Ok(())
+}
+
 async fn cmd_join(
     session: &mut SessionState,
     client: &node_service::Client,