docs: Add field visibility analysis and MIR BoxCall documentation

## Field Visibility Analysis Results
- Confirmed init{} fields are **public** in current Nyash implementation
- No access modifier (private/public) system currently implemented
- All fields accessible via me.fieldname syntax
- Documented findings for future reference

## MIR Documentation Enhancements
- Created comprehensive mir-dumper-guide.md for reading MIR dumps
- Enhanced mir-26-specification.md with BoxCall vs regular Call examples
- Added clear identification patterns:
  * BoxCall: `call %value.method(args)` (plugins/builtins)
  * Regular Call: `call %func(%me, args)` (user-defined boxes)

## VM Backend BoxRef Handling Improvements
- Fixed BoxRef method dispatch using share_box() instead of clone_box()
- Prevents unintended constructor calls during method resolution
- Maintains proper instance identity throughout VM execution

## MIR Builder User-Defined Box Tracking
- Added user_defined_boxes HashSet to track declared user boxes
- Improved method lowering decisions for user-defined vs builtin boxes
- Enhanced AST→MIR conversion accuracy for method calls

## Plugin Tester Lifecycle Enhancements
- Added comprehensive FileBox lifecycle testing (open/write/close)
- Enhanced cloneSelf() and copyFrom() testing with proper Handle parsing
- Added TLV encoding helpers for strings and bytes
- Improved error reporting and step-by-step validation

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

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

docs/reference/execution-backend/mir-26-specification.md

@@ -87,6 +87,21 @@ This document specifies the official 26-instruction set for Nyash MIR (Machine I
     %dst = call %box.method(%arg1, %arg2, ...)
     ```
     Effect: context-dependent
+    
+    **重要な識別方法：**
+    - BoxCall形式: `call %値.メソッド名(引数)` - 値（%7など）に対してメソッドを直接呼ぶ
+    - 通常のCall形式: `call %関数値(%me, 引数)` - 関数値を呼び、第1引数にmeを渡す
+    
+    例：
+    ```mir
+    ; BoxCall（プラグイン/ビルトインBoxのメソッド）
+    %17 = call %7.open(%14, %16)
+    %22 = call %7.write(%21)
+    
+    ; 通常のCall（ユーザー定義Boxのメソッド）
+    %func = const "UserBox.method/2"
+    %result = call %func(%me, %arg1)
+    ```
 
 13. **ExternCall** - Call external function
     ```mir