DOC: Example applications in documentation

docs/appdev/examples/04_tcp_client.md

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+<div align="center">
+  <h1>Example 04: TCP HTTP Client</h1>
+  <p><em>Utilizing lwIP to establish an outbound TCP connection.</em></p>
+</div>
+
+---
+
+This advanced example demonstrates the steps required to use the raw network system calls to establish a connection with an external HTTP server and dump the response over the terminal.
+
+## 📝 Concepts Introduced
+* Verifying the network state (`sys_network_is_initialized`, `sys_network_has_ip`).
+* Performing DNS lookups manually via `sys_dns_lookup`.
+* Managing strict TCP flow logic (`sys_tcp_connect`, send, block for receive).
+* Using the terminal `SYS_WRITE` output for debugging.
+
+---
+
+## 💻 The Code (`src/userland/cli/http_get.c`)
+
+```c
+#include <stdlib.h>
+#include <string.h>
+#include <syscall.h>
+
+int main(void) {
+  if (!sys_network_is_initialized() || !sys_network_has_ip()) {
+    printf("Network is unreachable! Make sure you inited the network first!\n");
+    return 1;
+  }
+
+  // 1. Resolve host name to IP
+  const char *target_host = "boreddev.nl";
+  net_ipv4_address_t server_ip;
+
+  printf("Resolving %s...\n", target_host);
+  if (sys_dns_lookup(target_host, &server_ip) < 0) {
+    printf("DNS Lookup failed.\n");
+    return 1;
+  }
+  printf("Resolved to: %d.%d.%d.%d\n", server_ip.bytes[0], server_ip.bytes[1],
+         server_ip.bytes[2], server_ip.bytes[3]);
+
+  // 2. Establish a TCP connection on port 80 (HTTP)
+  printf("Connecting...\n");
+  if (sys_tcp_connect(&server_ip, 80) < 0) {
+    printf("Connection failed.\n");
+    return 1;
+  }
+  printf("Connected! Sending GET request...\n");
+
+  // 3. Format and send the raw HTTP Request
+  char request[256];
+  strcpy(request, "GET / HTTP/1.1\r\nHost: ");
+  strcat(request, target_host);
+  strcat(request, "\r\nConnection: close\r\n\r\n");
+
+  if (sys_tcp_send(request, strlen(request)) < 0) {
+    printf("Failed to send data.\n");
+    sys_tcp_close();
+    return 1;
+  }
+
+  // 4. Block and wait for response data
+  char recv_buf[512];
+  int bytes_received;
+
+  printf("\n--- RESPONSE ---\n");
+  while ((bytes_received = sys_tcp_recv(recv_buf, sizeof(recv_buf) - 1)) > 0) {
+    recv_buf[bytes_received] = '\0'; // Null-terminate the chunk
+    printf("%s", recv_buf);          // Print the chunk to stdout
+  }
+
+  // 5. Cleanup
+  printf("\n--- END RESPONSE ---\n");
+  sys_tcp_close();
+  printf("Connection closed.\n");
+
+  return 0;
+}
+```
+
+## 🛠️ How it Works
+
+1.  **Network Setup**: First, we must ensure the host machine or QEMU environment gave BoredOS a valid IP address via DHCP. The `sys_network_has_ip()` check prevents our app from hanging trying to route data to nowhere.
+2.  **DNS (`sys_dns_lookup`)**: Since we want to connect to a domain name, not a raw IP, we query the DNS server configured by the OS (which it received via DHCP).
+3.  **Connection (`sys_tcp_connect`)**: We block the application thread while the OS performs the 3-way TCP handshake over port 80.
+4.  **Payload (`sys_tcp_send`)**: We format a compliant HTTP/1.1 payload representing a simple GET request for the root directory `/`.
+5.  **Chunked Receiving (`sys_tcp_recv`)**: The server's response might be larger than our `recv_buf` (512 bytes). Therefore, we loop. `sys_tcp_recv` blocks execution until data arrives. If it returns `0`, the remote server cleanly closed the connection (which happens automatically because we specified `Connection: close` in our request payload!).
+
+## 🚀 Running It
+
+Make sure QEMU is running with networking enabled. Launch the terminal and type `http_get`. You will see the raw headers and HTML source of the target webpage scroll down the CLI interface!