feat: add Turing Complete-style puzzle system

Add progressive puzzle mode alongside the existing sandbox:
- 8 levels from basic gates to 2-bit adder
- Truth table verification with pass/fail feedback
- Gate restrictions per level
- Custom components system (save circuits as reusable chips)
- Save/load circuits as JSON
- Level selection sidebar with difficulty ratings
- Mode toggle: Sandbox (free play) vs Puzzle (guided levels)

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

index.html

@@ -22,11 +22,18 @@
         <div class="separator"></div>
         <button class="action-btn sim-btn" id="sim-btn">Waveform</button>
         <div class="toolbar-right">
+            <button class="action-btn export-btn" id="export-btn" title="Export circuit">↓ Export</button>
+            <button class="action-btn import-btn" id="import-btn" title="Import circuit">↑ Import</button>
             <button class="action-btn help-btn" id="help-btn">? Help</button>
             <button class="action-btn" id="clear-btn">Clear All</button>
         </div>
     </div>
 
+    <!-- Hidden file input for import -->
+    <input type="file" id="import-file" accept=".json" style="display:none">
+
+    <!-- Mode toggle will be inserted here by puzzleUI.js -->
+
     <canvas id="canvas"></canvas>
 
     <div id="waveform-panel">