refactor(parser): Step 4 - Extract items module

- Create items module with global_vars, functions, static_items
- Move parse_global_var from mod.rs to items/global_vars.rs
- Move parse_function_declaration to items/functions.rs
- Move parse_static_declaration and parse_static_function to items/static_items.rs
- Clean up mod.rs by removing 197 lines of code
- Build passes successfully

Next: Step 5 - Final cleanup and documentation

src/parser/items/functions.rs

@@ -0,0 +1,79 @@
+/*!
+ * Function declaration parsing
+ */
+
+use crate::parser::{NyashParser, ParseError};
+use crate::parser::common::ParserUtils;
+use crate::tokenizer::TokenType;
+use crate::ast::{ASTNode, Span};
+use crate::must_advance;
+
+impl NyashParser {
+    /// function宣言をパース: function name(params) { body }
+    pub fn parse_function_declaration(&mut self) -> Result<ASTNode, ParseError> {
+        self.consume(TokenType::FUNCTION)?;
+        
+        // 関数名を取得
+        let name = if let TokenType::IDENTIFIER(name) = &self.current_token().token_type {
+            let name = name.clone();
+            self.advance();
+            name
+        } else {
+            let line = self.current_token().line;
+            return Err(ParseError::UnexpectedToken {
+                found: self.current_token().token_type.clone(),
+                expected: "function name".to_string(),
+                line,
+            });
+        };
+        
+        // パラメータリストをパース
+        self.consume(TokenType::LPAREN)?;
+        let mut params = Vec::new();
+        
+        while !self.match_token(&TokenType::RPAREN) && !self.is_at_end() {
+            must_advance!(self, _unused, "function declaration parameter parsing");
+            
+            if let TokenType::IDENTIFIER(param) = &self.current_token().token_type {
+                params.push(param.clone());
+                self.advance();
+                
+                if self.match_token(&TokenType::COMMA) {
+                    self.advance();
+                }
+            } else if !self.match_token(&TokenType::RPAREN) {
+                let line = self.current_token().line;
+                return Err(ParseError::UnexpectedToken {
+                    found: self.current_token().token_type.clone(),
+                    expected: "parameter name".to_string(),
+                    line,
+                });
+            }
+        }
+        
+        self.consume(TokenType::RPAREN)?;
+        
+        // 関数本体をパース
+        self.consume(TokenType::LBRACE)?;
+        self.skip_newlines();
+        
+        let mut body = Vec::new();
+        while !self.match_token(&TokenType::RBRACE) && !self.is_at_end() {
+            self.skip_newlines();
+            if !self.match_token(&TokenType::RBRACE) {
+                body.push(self.parse_statement()?);
+            }
+        }
+        
+        self.consume(TokenType::RBRACE)?;
+        
+        Ok(ASTNode::FunctionDeclaration {
+            name,
+            params,
+            body,
+            is_static: false,  // 通常の関数は静的でない
+            is_override: false, // デフォルトは非オーバーライド
+            span: Span::unknown(),
+        })
+    }
+}