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/saveLoad.js

@@ -0,0 +1,149 @@
+// Save/Load system — export and import circuits and progress
+import { state } from './state.js';
+import { progress } from './levels.js';
+
+/**
+ * Save complete application state to JSON
+ */
+export function saveState() {
+    return {
+        timestamp: new Date().toISOString(),
+        circuit: {
+            gates: state.gates,
+            connections: state.connections,
+            nextId: state.nextId
+        },
+        camera: {
+            camX: state.camX,
+            camY: state.camY,
+            zoom: state.zoom
+        },
+        components: state.customComponents || {},
+        progress: {
+            unlockedLevels: progress.unlockedLevels,
+            completedLevels: progress.completedLevels,
+            currentLevel: progress.currentLevel,
+            customComponents: progress.customComponents
+        }
+    };
+}
+
+/**
+ * Load application state from JSON
+ */
+export function loadState(data) {
+    if (!data || !data.circuit) {
+        return { success: false, error: 'Invalid save data' };
+    }
+
+    try {
+        // Load circuit
+        state.gates = JSON.parse(JSON.stringify(data.circuit.gates));
+        state.connections = JSON.parse(JSON.stringify(data.circuit.connections));
+        state.nextId = data.circuit.nextId;
+
+        // Load camera
+        if (data.camera) {
+            state.camX = data.camera.camX;
+            state.camY = data.camera.camY;
+            state.zoom = data.camera.zoom;
+        }
+
+        // Load components
+        if (data.components) {
+            state.customComponents = JSON.parse(JSON.stringify(data.components));
+        }
+
+        // Load progress
+        if (data.progress) {
+            progress.unlockedLevels = data.progress.unlockedLevels || ['buffer'];
+            progress.completedLevels = data.progress.completedLevels || [];
+            progress.currentLevel = data.progress.currentLevel || null;
+            progress.customComponents = data.progress.customComponents || {};
+        }
+
+        return { success: true };
+    } catch (e) {
+        return { success: false, error: e.message };
+    }
+}
+
+/**
+ * Export as JSON file (for download)
+ */
+export function exportAsFile(filename = 'logic-circuit.json') {
+    const data = saveState();
+    const json = JSON.stringify(data, null, 2);
+    const blob = new Blob([json], { type: 'application/json' });
+    const url = URL.createObjectURL(blob);
+    const a = document.createElement('a');
+    a.href = url;
+    a.download = filename;
+    a.click();
+    URL.revokeObjectURL(url);
+}
+
+/**
+ * Import from file (returns Promise)
+ */
+export function importFromFile(file) {
+    return new Promise((resolve, reject) => {
+        const reader = new FileReader();
+        reader.onload = (e) => {
+            try {
+                const data = JSON.parse(e.target.result);
+                const result = loadState(data);
+                resolve(result);
+            } catch (err) {
+                reject(new Error('Failed to parse JSON'));
+            }
+        };
+        reader.onerror = () => reject(new Error('Failed to read file'));
+        reader.readAsText(file);
+    });
+}
+
+/**
+ * Export circuit as a simple netlist (text format)
+ */
+export function exportAsNetlist() {
+    let netlist = '# Logic Circuit Netlist\n\n';
+    netlist += '## Gates\n';
+
+    state.gates.forEach(gate => {
+        netlist += `${gate.type} ${gate.id} at (${gate.x.toFixed(0)}, ${gate.y.toFixed(0)})\n`;
+    });
+
+    netlist += '\n## Connections\n';
+    state.connections.forEach(conn => {
+        netlist += `${conn.from}:${conn.fromPort} -> ${conn.to}:${conn.toPort}\n`;
+    });
+
+    return netlist;
+}
+
+/**
+ * Copy state to clipboard as JSON (useful for sharing)
+ */
+export function copyToClipboard() {
+    const data = saveState();
+    const json = JSON.stringify(data);
+    navigator.clipboard.writeText(json).then(() => {
+        alert('State copied to clipboard');
+    }).catch(() => {
+        alert('Failed to copy to clipboard');
+    });
+}
+
+/**
+ * Paste state from clipboard
+ */
+export async function pasteFromClipboard() {
+    try {
+        const text = await navigator.clipboard.readText();
+        const data = JSON.parse(text);
+        return loadState(data);
+    } catch (e) {
+        return { success: false, error: e.message };
+    }
+}