fix: Phase 9.78e+: Fix birth constructor and builtin Box method inheritance

🌟 Major fixes:
- Fix birth constructor lookup (was searching for 'init/N', now 'birth/N')
- Fix builtin Box method inheritance via __builtin_content field
- Remove InstanceBox wrapping for builtin Boxes
- Update execute_builtin_birth_method to properly store builtin content
- Update method resolution to check __builtin_content for builtin methods

✅ All tests pass:
- Birth constructors work correctly
- User-to-user Box delegation works
- User-to-builtin Box delegation works (toString() properly inherited)
- Apps folder tested (chip8, kilo editor run successfully)

🤖 Generated with [Claude Code](https://claude.ai/code)

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

local_tests/test_builtin_delegation.nyash

@@ -0,0 +1,30 @@
+// 🎯 ビルトインBox継承透過テスト
+
+box EnhancedString from StringBox {
+    init { prefix, suffix }
+    
+    birth(text) {
+        from StringBox.birth(text)  // 🔥 これが透過的にpackに変換される
+        me.prefix = "【"
+        me.suffix = "】"
+        print("📝 EnhancedString誕生: " + me.prefix + text + me.suffix)
+    }
+    
+    enhanced() {
+        return me.prefix + me.toString() + me.suffix + "✨"
+    }
+}
+
+static box Main {
+    init { console }
+    
+    main() {
+        me.console = new ConsoleBox()
+        me.console.log("🧪 ビルトインBox継承透過テスト開始")
+        
+        local enhanced = new EnhancedString("Hello")
+        print("結果: " + enhanced.enhanced())
+        
+        return "透過テスト完了"
+    }
+}