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/state.js

@@ -33,5 +33,8 @@ export const state = {
     // Simulation
     simRunning: false,
     simInterval: null,
-    simSpeed: 500        // ms per tick
+    simSpeed: 500,       // ms per tick
+
+    // Puzzle/Components
+    customComponents: {}  // { id -> component definition }
 };