fix: Capture correct display output in g.drawString mock

Co-authored-by: aider (gemini/gemini-2.5-pro) <aider@aider.chat>

calculator/test.js

@@ -5,6 +5,7 @@ const assert = require('assert');
 // --- Mock Bangle.js environment ---
 let mock_display_str = "";
 let mock_ui_callbacks = {};
+let drawn_since_clear = false;
 
 // Mock 'g' (graphics)
 global.g = {
@@ -12,8 +13,8 @@ global.g = {
   _bg: 0,
   _font: "6x8",
   _font_size: 1,
-  clear: () => {},
-  clearRect: () => {},
+  clear: () => { drawn_since_clear = false; },
+  clearRect: () => { drawn_since_clear = false; },
   setColor: (c) => { global.g._col = c; return global.g; },
   setBgColor: (c) => { global.g._bg = c; return global.g; },
   fillRect: () => {},
@@ -32,7 +33,12 @@ global.g = {
   stringWidth: (s) => s.length * 8 * (global.g._font_size || 1), // simple approximation
   getWidth: () => 176,
   getHeight: () => 176,
-  drawString: (s) => { mock_display_str = String(s); },
+  drawString: (s) => {
+    if (!drawn_since_clear) {
+      mock_display_str = String(s);
+      drawn_since_clear = true;
+    }
+  },
   getFontHeight: () => 8 * (global.g._font_size || 1)
 };