feat(phase-9.75e): Complete using nyashstd standard library implementation

🎉 Phase 9.75e完了: using nyashstdシステム完全実装成功

✅ 実装完了項目:
- USING tokenizer integration: TokenType::USING token support
- UsingStatement AST node: Complete using statement parsing
- BuiltinStdlib infrastructure: Core standard library framework
- Full interpreter integration: Complete namespace resolution
- Module system integration: Fixed crate::stdlib import issues

🌟 動作確認済み標準ライブラリ機能:
- string.create("text") → StringBox creation
- string.upper(str) → Uppercase string conversion
- integer.create(42) → IntegerBox creation
- bool.create(true) → BoolBox creation
- array.create() → Empty ArrayBox creation
- console.log("message") → Console output

📋 実装ファイル:
- src/tokenizer.rs: USING token support
- src/ast.rs: UsingStatement AST node
- src/parser/statements.rs: using statement parser
- src/interpreter/statements.rs: using statement execution
- src/interpreter/core.rs: stdlib namespace resolution
- src/stdlib/mod.rs: Complete BuiltinStdlib implementation
- src/lib.rs + src/main.rs: Module declaration integration

🎯 テスト成功:
All nyashstd functions work perfectly with comprehensive test coverage.
Local test file: local_tests/test_nyashstd.nyash

Everything is Box哲学を維持しながらモダンな標準ライブラリアクセスを実現\!

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

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

src/parser/statements.rs

@@ -62,6 +62,9 @@ impl NyashParser {
             TokenType::THROW => {
                 self.parse_throw()
             },
+            TokenType::USING => {
+                self.parse_using()
+            },
             TokenType::FROM => {
                 // 🔥 from構文: from Parent.method(args) または from Parent.constructor(args)
                 self.parse_from_call_statement()
@@ -455,4 +458,32 @@ impl NyashParser {
         // 例: from Animal.constructor() （戻り値を使わない）
         Ok(from_call_expr)
     }
+    
+    /// using文をパース: using namespace_name
+    pub(super) fn parse_using(&mut self) -> Result<ASTNode, ParseError> {
+        self.advance(); // consume 'using'
+        
+        // 名前空間名を取得
+        if let TokenType::IDENTIFIER(namespace_name) = &self.current_token().token_type {
+            let name = namespace_name.clone();
+            self.advance();
+            
+            // Phase 0では "nyashstd" のみ許可
+            if name != "nyashstd" {
+                return Err(ParseError::UnsupportedNamespace { 
+                    name, 
+                    line: self.current_token().line 
+                });
+            }
+            
+            Ok(ASTNode::UsingStatement {
+                namespace_name: name,
+                span: Span::unknown(),
+            })
+        } else {
+            Err(ParseError::ExpectedIdentifier { 
+                line: self.current_token().line 
+            })
+        }
+    }
 }