feat(client): v2 CLI binary over SDK with simplified commands

Add v2_main.rs and v2_commands.rs as thin wrappers over QpqClient from
quicproquo-sdk. Activated via --features v2 (feature-gated so v1 main.rs
remains the default). Includes auto-server-launch, clap-based command
surface (register-user, login, whoami, health, resolve, dm, group,
devices), and ClientConfig-based connection setup. MLS-dependent commands
(send, recv, group create/invite) print stubs pointing to the REPL.

crates/quicproquo-client/src/main.rs

@@ -1,20 +1,35 @@
 //! quicproquo CLI client.
 
+// ── v2 feature gate: when compiled with --features v2, use the SDK-based CLI.
+#[cfg(feature = "v2")]
+mod v2_commands;
+#[cfg(feature = "v2")]
+mod v2_main;
+
+#[cfg(feature = "v2")]
+fn main() {
+    v2_main::main();
+}
+
+// ── v1 CLI (default) ─────────────────────────────────────────────────────────
+#[cfg(not(feature = "v2"))]
 use std::path::{Path, PathBuf};
-
+#[cfg(not(feature = "v2"))]
 use anyhow::Context;
+#[cfg(not(feature = "v2"))]
 use clap::{Parser, Subcommand};
-
+#[cfg(not(feature = "v2"))]
 use quicproquo_client::{
     cmd_chat, cmd_check_key, cmd_create_group, cmd_demo_group, cmd_export, cmd_export_verify,
     cmd_fetch_key, cmd_health, cmd_invite, cmd_join, cmd_login, cmd_ping, cmd_recv, cmd_register,
     cmd_register_state, cmd_refresh_keypackage, cmd_register_user, cmd_send, cmd_whoami,
     init_auth, run_repl, set_insecure_skip_verify, ClientAuth,
 };
-#[cfg(feature = "tui")]
+#[cfg(all(feature = "tui", not(feature = "v2")))]
 use quicproquo_client::client::tui::run_tui;
 
 // ── CLI ───────────────────────────────────────────────────────────────────────
+#[cfg(not(feature = "v2"))]
 
 #[derive(Debug, Parser)]
 #[command(name = "qpq", about = "quicproquo CLI client", version)]
@@ -90,6 +105,7 @@ struct Args {
     command: Option<Command>,
 }
 
+#[cfg(not(feature = "v2"))]
 #[derive(Debug, Subcommand)]
 enum Command {
     /// Register a new user via OPAQUE (password never leaves the client).
@@ -424,7 +440,7 @@ enum Command {
 }
 
 // ── Helpers ───────────────────────────────────────────────────────────────────
-
+#[cfg(not(feature = "v2"))]
 /// Returns `qpq-{username}.bin` when `state` is still at the default
 /// (`qpq-state.bin`) and a username has been provided. Otherwise returns
 /// `state` unchanged. This lets `qpq --username alice` automatically isolate
@@ -440,7 +456,7 @@ fn derive_state_path(state: PathBuf, username: Option<&str>) -> PathBuf {
 
 // ── Playbook execution ───────────────────────────────────────────────────────
 
-#[cfg(feature = "playbook")]
+#[cfg(all(feature = "playbook", not(feature = "v2")))]
 async fn run_playbook(
     playbook_path: &Path,
     state: &Path,
@@ -511,6 +527,7 @@ async fn run_playbook(
 
 // ── Entry point ───────────────────────────────────────────────────────────────
 
+#[cfg(not(feature = "v2"))]
 #[tokio::main]
 async fn main() -> anyhow::Result<()> {
     // Install the rustls crypto provider before any TLS operations.