Add dual-stack IPv4+IPv6 listening

Server now binds on both IPv4 (0.0.0.0) and IPv6 (::) by default.
Uses tokio::select! to accept from whichever listener has a connection.

New flags:
  --listen <addr>   IPv4 listen address (default: 0.0.0.0, "none" to disable)
  --listen6 <addr>  IPv6 listen address (default: ::, "none" to disable)

Examples:
  btest -s                          # listen on both v4 and v6
  btest -s --listen6 none           # IPv4 only
  btest -s --listen none            # IPv6 only
  btest -s --listen 192.168.1.1     # specific IPv4 address

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

tests/ecsrp5_test.rs

@@ -10,7 +10,9 @@ async fn start_ecsrp5_server(port: u16) {
             port,
             Some("testuser".into()),
             Some("testpass".into()),
-            true, // ecsrp5
+            true,
+            Some("127.0.0.1".into()),
+            None,
         )
         .await;
     });
@@ -23,7 +25,9 @@ async fn start_md5_server(port: u16) {
             port,
             Some("testuser".into()),
             Some("testpass".into()),
-            false, // md5
+            false,
+            Some("127.0.0.1".into()),
+            None,
         )
         .await;
     });
@@ -32,7 +36,7 @@ async fn start_md5_server(port: u16) {
 
 async fn start_noauth_server(port: u16) {
     tokio::spawn(async move {
-        let _ = btest_rs::server::run_server(port, None, None, false).await;
+        let _ = btest_rs::server::run_server(port, None, None, false, Some("127.0.0.1".into()), None).await;
     });
     tokio::time::sleep(Duration::from_millis(200)).await;
 }

tests/integration_test.rs

@@ -8,7 +8,7 @@ async fn start_test_server(port: u16, auth_user: Option<&str>, auth_pass: Option
     let user = auth_user.map(String::from);
     let pass = auth_pass.map(String::from);
     tokio::spawn(async move {
-        let _ = btest_rs::server::run_server(port, user, pass, false).await;
+        let _ = btest_rs::server::run_server(port, user, pass, false, Some("127.0.0.1".into()), None).await;
     });
     tokio::time::sleep(Duration::from_millis(100)).await;
 }