v0.5.0: IPv6 off by default, mark as experimental

IPv6 listener now requires explicit --listen6 flag (disabled by default).
TCP over IPv6 works fully. UDP over IPv6 has macOS kernel limitations
(ENOBUFS on send_to). On Linux, IPv6 UDP works fine.

Usage:
  btest -s                    # IPv4 only (default)
  btest -s --listen6          # IPv4 + IPv6 on ::
  btest -s --listen6 ::1      # IPv4 + IPv6 on specific address

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.rs

@@ -54,9 +54,9 @@ struct Cli {
     #[arg(long = "listen", default_value = "0.0.0.0")]
     listen_addr: String,
 
-    /// Listen address for IPv6 (default: ::, use "none" to disable)
-    #[arg(long = "listen6", default_value = "::")]
-    listen6_addr: String,
+    /// Enable IPv6 listener (experimental — TCP works, UDP has issues on macOS)
+    #[arg(long = "listen6", default_missing_value = "::", num_args = 0..=1)]
+    listen6_addr: Option<String>,
 
     /// Authentication username
     #[arg(short = 'a', long = "authuser")]
@@ -110,7 +110,7 @@ async fn main() -> anyhow::Result<()> {
     if cli.server {
         // Server mode
         let v4 = if cli.listen_addr.eq_ignore_ascii_case("none") { None } else { Some(cli.listen_addr) };
-        let v6 = if cli.listen6_addr.eq_ignore_ascii_case("none") { None } else { Some(cli.listen6_addr) };
+        let v6 = cli.listen6_addr; // None unless --listen6 is passed
         tracing::info!("Starting btest server on port {}", cli.port);
         server::run_server(cli.port, cli.auth_user, cli.auth_pass, cli.ecsrp5, v4, v6).await?;
     } else if let Some(host) = cli.client {