Reduce UART buffer flush threshold from 512 to 64 bytes

Prevents MAVLink frames from being split at the 512-byte boundary
into two UDP packets. With 64B threshold + 3ms inactivity timeout,
each MAVLink frame (9-280 bytes) fits in a single UDP packet.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.cpp

@@ -41,7 +41,7 @@ uint32_t bytesUdpToUart = 0;
 unsigned long lastStatsTime = 0;
 
 // ── Buffer UART → UDP ────────────────────────────────────
-#define UART_BUF_SIZE   512
+#define UART_BUF_SIZE   64
 #define UART_FLUSH_MS   3
 uint8_t uartBuf[UART_BUF_SIZE];
 int uartBufLen = 0;