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>

js/events.js

@@ -5,6 +5,9 @@ import { evaluateAll, findGateAt, findPortAt } from './gates.js';
 import { manualStep, clearWaveData } from './waveform.js';
 import { startSim, stopSim, adjustSpeed } from './simulation.js';
 import { resize, screenToWorld } from './renderer.js';
+import { puzzleMode, currentLevel, showLevelPanel } from './puzzleUI.js';
+import { getLevel } from './levels.js';
+import { saveState, loadState, exportAsFile, importFromFile } from './saveLoad.js';
 
 const PAN_SPEED = 40;
 
@@ -176,7 +179,18 @@ export function initEvents() {
 
     // ==================== TOOLBAR ====================
     document.querySelectorAll('.gate-btn').forEach(btn => {
-        btn.addEventListener('click', () => { state.placingGate = btn.dataset.gate; });
+        btn.addEventListener('click', () => {
+            const gateName = btn.dataset.gate;
+            // In puzzle mode, check if gate is allowed
+            if (puzzleMode && currentLevel) {
+                const level = getLevel(currentLevel.id);
+                if (!level.availableGates.includes(gateName)) {
+                    alert(`${gateName} is not available in this level.`);
+                    return;
+                }
+            }
+            state.placingGate = gateName;
+        });
     });
 
     document.getElementById('clear-btn').addEventListener('click', () => {
@@ -189,6 +203,33 @@ export function initEvents() {
         }
     });
 
+    // Export/Import
+    document.getElementById('export-btn').addEventListener('click', () => {
+        exportAsFile(`circuit-${new Date().toISOString().slice(0, 10)}.json`);
+    });
+
+    document.getElementById('import-btn').addEventListener('click', () => {
+        document.getElementById('import-file').click();
+    });
+
+    document.getElementById('import-file').addEventListener('change', async (e) => {
+        const file = e.target.files[0];
+        if (!file) return;
+        try {
+            const result = await importFromFile(file);
+            if (result.success) {
+                alert('Circuit imported successfully!');
+                evaluateAll();
+            } else {
+                alert(`Import failed: ${result.error}`);
+            }
+        } catch (err) {
+            alert(`Import error: ${err.message}`);
+        }
+        // Reset file input
+        e.target.value = '';
+    });
+
     // Help modal
     document.getElementById('help-btn').addEventListener('click', () => {
         document.getElementById('help-modal').classList.add('visible');