DOC: Example applications in documentation

docs/appdev/examples/02_basic_window.md

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+<div align="center">
+  <h1>Example 02: Basic Window</h1>
+  <p><em>An introduction to libui and creating graphical apps.</em></p>
+</div>
+
+---
+
+This example demonstrates how to create an empty window that stays active on the screen until the user explicitly closes it by clicking the 'X' button.
+
+## 📝 Concepts Introduced
+* Including `libui.h` and the event structure.
+* Creating a `ui_window_t` handle.
+* Creating an infinite event loop using `ui_get_event()`.
+* Yielding CPU time to the kernel via `sys_yield()`.
+
+---
+
+## 💻 The Code (`src/userland/gui/basic_window.c`)
+
+```c
+#include <stdlib.h>
+#include <libui.h>
+#include <syscall.h>
+
+int main(void) {
+    // 1. Ask the Window Manager to create a new window
+    // Arguments are: Title, X Position, Y Position, Width, Height
+    ui_window_t wid = ui_window_create("My First GUI", 100, 100, 400, 300);
+    
+    if (wid < 0) {
+        printf("Failed to create the window!\n");
+        return 1;
+    }
+
+    // 2. Define our event object
+    gui_event_t event;
+
+    // 3. Enter the main event loop
+    while (1) {
+        // ui_get_event is non-blocking. It returns true if an event was waiting.
+        if (ui_get_event(wid, &event)) {
+            
+            // Check what type of event occurred
+            if (event.type == GUI_EVENT_CLOSE) {
+                // The user clicked the 'X' button in the titlebar!
+                printf("Window closed cleanly by user.\n");
+                break; // Break the infinite loop
+            }
+        }
+        
+        // 4. CRITICAL: Yield the remainder of our timeslice
+        // If we don't do this, the while(1) loop will consume 100% of the CPU
+        // and starve the rest of the OS!
+        sys_yield();
+    }
+
+    // Returning from main will automatically destroy the window and exit the process.
+    return 0;
+}
+```
+
+## 🛠️ How it Works
+
+1.  **Window Handle (`wid`)**: `ui_window_create` sends a request to the kernel. The kernel allocates the memory for the window and returns a numerical ID (the handle) that we use for all future interactions with that specific window.
+2.  **The Event Loop**: Graphical programs run forever until closed. The `while (1)` loop serves this purpose.
+3.  **Polling**: `ui_get_event` asks the kernel, "Hey, did the user click my window or press a key since the last time I asked?". It is non-blocking, so it immediately returns `false` if nothing happened.
+4.  **CPU Yielding**: Since we are constantly polling in a tight loop, we call `sys_yield()` at the end of the loop frame. This politely tells the OS scheduler, "I'm done checking for events, go ahead and let another program run for a bit."
+
+## 🚀 Running It
+
+Launch the Terminal and type `basic_window`. You'll see an empty window appear that you can move around the screen!