feat(constructor): implement birth() syntax and pack transparency system

## 🌟 birth() Constructor Implementation
- Add BIRTH token to tokenizer (src/tokenizer.rs:37,403)
- Implement birth() parser support (src/parser/mod.rs)
- Add birth() interpreter support with priority system
- Priority: birth > pack > init > Box名

## 🚨 Fix Documentation Inconsistencies
- Fix delegation-system.md: pack → birth unified
- Fix box-design/README.md: add pack-specific section
- Fix LANGUAGE_GUIDE.md: birth unified, pack builtin-only
- Fix CLAUDE.md: birth philosophy, pack system separation

## 📋 pack Transparency System Design
- Create phase_8_8_pack_transparency_system.md specification
- Establish correct pack definition: builtin Box inheritance only
- Design user-transparent system: from BuiltinBox() → internal pack
- Comprehensive test cases and implementation requirements

## 🧪 Testing
- Add test_birth_simple.nyash: birth() functionality verification
- Document constructor name decision process
- Prepare for Copilot implementation with clear specifications

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

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

src/tokenizer.rs

@@ -33,7 +33,8 @@ pub enum TokenType {
     THIS,
     ME,
     INIT,            // init (初期化ブロック)
-    PACK,            // pack (コンストラクタ)
+    PACK,            // pack (コンストラクタ - 互換性)
+    BIRTH,           // birth (コンストラクタ)
     NOWAIT,          // nowait
     AWAIT,           // await
     INTERFACE,       // interface
@@ -399,6 +400,7 @@ impl NyashTokenizer {
             "me" => TokenType::ME,
             "init" => TokenType::INIT,
             "pack" => TokenType::PACK,
+            "birth" => TokenType::BIRTH,
             "nowait" => TokenType::NOWAIT,
             "await" => TokenType::AWAIT,
             "interface" => TokenType::INTERFACE,