Add client syslog events, fix client UDP TX error threshold

- Client mode now emits TEST_START and TEST_END syslog events
- Client UDP TX threshold raised from 1000 to 50000 with adaptive backoff
  (matching server behavior) — prevents premature TX death on macOS
- Updated all docs (README, user-guide, architecture, protocol, docker)
- Added results.csv to gitignore

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

src/client.rs

@@ -289,13 +289,19 @@ async fn udp_client_tx_loop(
                 state.tx_bytes.fetch_add(n as u64, Ordering::Relaxed);
                 consecutive_errors = 0;
             }
-            Err(_) => {
+            Err(e) => {
                 consecutive_errors += 1;
-                if consecutive_errors > 1000 {
+                if consecutive_errors == 1 {
+                    tracing::debug!("UDP TX send error: {} (target)", e);
+                }
+                if consecutive_errors > 50000 {
                     tracing::warn!("UDP TX: too many consecutive send errors, stopping");
                     break;
                 }
-                tokio::time::sleep(Duration::from_micros(200)).await;
+                let backoff = Duration::from_micros(
+                    (200 + consecutive_errors.min(5000) as u64 * 10).min(10000)
+                );
+                tokio::time::sleep(backoff).await;
                 continue;
             }
         }

src/main.rs

@@ -172,6 +172,9 @@ async fn main() -> anyhow::Result<()> {
         };
         let proto_str = if cli.udp { "UDP" } else { "TCP" };
 
+        // Log test start
+        syslog_logger::test_start(&host, proto_str, dir_str, 0);
+
         // Run client with optional duration timeout
         let start = std::time::Instant::now();
         let client_fut = client::run_client(
@@ -204,10 +207,15 @@ async fn main() -> anyhow::Result<()> {
 
         let elapsed = start.elapsed().as_secs();
 
+        // Log test end to syslog
+        syslog_logger::test_end(
+            &host, proto_str, dir_str,
+            0, 0, 0, elapsed as u32,
+        );
+
         // Write CSV if enabled
         if csv_output::is_enabled() {
             let auth_type = if cli.auth_user.is_some() { "auth" } else { "none" };
-            // For client mode we don't track detailed stats yet, but duration is useful
             csv_output::write_result(
                 &host, cli.port, proto_str, dir_str,
                 elapsed, 0, 0, 0, auth_type,