ui: document anchor property and demo top-left anchor

- reference.md: document UIObject.anchor in the UI System section
- examples/uiSystem/game.js: add a 'Menu' toggle button anchored to the
  top-left corner of the canvas with a 20px inset; showcases the
  anchor + self-pivot combination

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: KilledByAPixel

examples/uiSystem/game.js

@@ -88,6 +88,11 @@ function createUI()
         ()=> setMenuVisible(false));
     button2.navigationIndex = ++navigationIndex;
     button2.navigationAutoSelect = true;
+
+    // menu toggle button anchored to top-left corner of the canvas with a 20px inset
+    const buttonToggle = new LJS.UIButton(vec2(20, 20), vec2(150, 60), 'Menu');
+    buttonToggle.anchor = vec2(-1, -1);
+    buttonToggle.onClick = ()=> setMenuVisible(!getMenuVisible());
 }
 
 ///////////////////////////////////////////////////////////////////////////////

reference.md

@@ -523,6 +523,7 @@ uiSystem.drawLine(posA, posB, lineWidth, lineColor)
 
 // Base widget
 new UIObject(pos=vec2(), size=vec2())
+UIObject.anchor                        // vec2 in [-1,1]; anchors to parent (or canvas if root) + self-pivot; default vec2()=center
 UIObject.addChild(child)               // Returns child, parents it
 UIObject.removeChild(child)
 UIObject.destroy()