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>

src/ast.rs

@@ -460,6 +460,10 @@ pub enum ASTNode {
     BoxDeclaration {
         name: String,
         fields: Vec<String>,
+        /// 公開フィールド（public { ... }）
+        public_fields: Vec<String>,
+        /// 非公開フィールド（private { ... }）
+        private_fields: Vec<String>,
         methods: HashMap<String, ASTNode>, // method_name -> FunctionDeclaration
         constructors: HashMap<String, ASTNode>, // constructor_key -> FunctionDeclaration
         init_fields: Vec<String>,         // initブロック内のフィールド定義