refactor(parser): Step 3 - Extract declarations module

- Create declarations module with box_definition, static_box, dependency_helpers
- Move parse_box_declaration, parse_interface_box_declaration from mod.rs
- Move parse_static_box and dependency checking logic
- Temporarily restore parse_global_var, parse_function_declaration, parse_static_declaration to mod.rs
- Fix visibility issues (pub vs pub(super))
- Fix TokenType::IMPLEMENTS not defined issue
- Fix Loop AST node condition type mismatch
- Build passes successfully

Next: Step 4 - Extract items module for global_vars, functions, static_items

src/parser/declarations/mod.rs

@@ -0,0 +1,15 @@
+/*!
+ * Parser Declarations Module
+ * 
+ * 宣言（Declaration）の解析を担当するモジュール群
+ * Box定義、関数定義、use文などの宣言を処理
+ */
+
+pub mod box_definition;
+pub mod static_box;
+pub mod dependency_helpers;
+
+// Re-export commonly used items
+pub use box_definition::*;
+pub use static_box::*;
+pub use dependency_helpers::*;