feat: Implement field visibility (public/private) system

## Major Features Added

### Field Visibility System
- Added `private { ... }` and `public { ... }` blocks in box declarations
- Default visibility is now handled explicitly (fields must be in either block)
- Visibility checks enforced at both interpreter and VM levels

### Parser Enhancements
- Extended AST with public_fields and private_fields vectors
- Added parsing for visibility blocks in box definitions
- Maintained backward compatibility with existing `init { ... }` syntax

### Interpreter Implementation
- Added visibility checks in field access (get_field/set_field)
- External access to private fields now throws appropriate errors
- Internal access (within methods) always allowed

### VM Implementation
- Extended VM with object_class tracking for visibility checks
- RefGet/RefSet instructions now enforce field visibility
- Fixed nested box declaration collection (boxes defined inside methods)

### Test Examples Added
- docs/examples/visibility_ok.nyash - demonstrates correct usage
- docs/examples/visibility_error.nyash - tests private field access errors

## Technical Details

### Error Messages
- Interpreter: "Field 'X' is private in Y"
- VM: Same error message for consistency

### Current Limitations
- All RefGet/RefSet treated as external access in VM (internal flag future work)
- Legacy `init { ... }` fields treated as having unspecified visibility

## Test Results
✅ Interpreter: Both test cases pass correctly
✅ VM: Both test cases pass correctly after nested declaration fix

This implements the foundation for proper encapsulation in Nyash,
following the "explicit is better than implicit" philosophy.

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

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

docs/examples/visibility_error.nyash

@@ -0,0 +1,27 @@
+static box Main {
+    init { console }
+
+    main() {
+        me.console = new ConsoleBox()
+        
+        box User {
+            private { age, passwordHash }
+            public  { name }
+
+            init { name, age }
+
+            birth(n, a) {
+                me.name = n
+                me.age = a
+            }
+        }
+
+        local u = new User("Alice", 20)
+        me.console.log("name(public)=" + u.name)   # OK: public
+        
+        # 以下は外部からprivateフィールドへのアクセス → エラーになる想定
+        u.age = 30                 # 外部からの代入（NG）
+        me.console.log(u.age)      # 外部からの参照（NG）
+    }
+}
+