Performance and stability pass (v1.5.9)

- Tighten transport announce rate limit from 5/sec to 2/sec
- Reduce RNS loop frequency from 200Hz to 100Hz
- Throttle LVGL rendering to 30 FPS (was unthrottled)
- Time-box TCP frame loop to 3 frames / 8ms per client
- Add 12ms global TCP budget across all clients
- Replace blocking WiFi announce delay(1500) with deferred non-blocking check
- Rotate persistData() across 3 cycles to spread file I/O
- Reduce LvNodesScreen rebuild frequency (2s → 5s, skip small transient changes)
- Bump version to 1.5.9

src/transport/TCPClientInterface.cpp

@@ -71,9 +71,9 @@ void TCPClientInterface::loop() {
         return;  // Will reconnect on next loop iteration
     }
 
-    // Drain multiple incoming frames per loop (up to 10, time-boxed)
+    // Drain multiple incoming frames per loop (up to 3, time-boxed)
     unsigned long tcpStart = millis();
-    for (int i = 0; i < 10 && _client.available() && (millis() - tcpStart < TCP_LOOP_BUDGET_MS); i++) {
+    for (int i = 0; i < 3 && _client.available() && (millis() - tcpStart < TCP_LOOP_BUDGET_MS); i++) {
         unsigned long rxStart = millis();
         int len = readFrame();
         if (len > 0) {