refactor: MIR builder modularization complete - ready for handoff

- MIRビルダーのモジュール化完了（1,547行→6モジュール）
  - core.rs (205行): MirBuilder本体
  - expressions.rs (621行): 式変換処理
  - statements.rs (165行): 文変換処理
  - control_flow.rs (194行): 制御フロー
  - box_handlers.rs (73行): Box処理
- 現在builder_modularizedに退避（MIR命令構造変更により調整必要）
- フルビルド可能な状態を維持
- CURRENT_TASK.mdに引き継ぎポイント記載

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

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

src/mir/builder_modularized/mod.rs

@@ -0,0 +1,27 @@
+/*!
+ * MIR Builder Module - Modular AST to MIR conversion
+ * 
+ * This module contains the refactored MIR builder split into focused sub-modules:
+ * 
+ * - `core`: Core builder functionality and instruction emission
+ * - `expressions`: Expression AST node conversion 
+ * - `statements`: Statement AST node conversion
+ * - `control_flow`: Control flow constructs (if, loop, try-catch)
+ * - `box_handlers`: Box-related operations (new, declarations)
+ */
+
+pub mod core;
+pub mod expressions;
+pub mod statements; 
+pub mod control_flow;
+pub mod box_handlers;
+
+// Re-export the main builder struct and key functionality
+pub use self::core::MirBuilder;
+
+// Re-export commonly used types from the parent module
+pub use super::{
+    MirInstruction, BasicBlock, BasicBlockId, MirFunction, MirModule,
+    FunctionSignature, ValueId, ConstValue, BinaryOp, UnaryOp, CompareOp,
+    MirType, EffectMask, Effect, BasicBlockIdGenerator, ValueIdGenerator
+};