refactor(vm): Phase 8 - Debug Trace Macro統一化（12行削減）

trace_dispatch!マクロで6箇所のVM_TRACEパターンを統一

実装内容:
- mod.rsにtrace_dispatch!マクロ定義
- boxes.rs: 6箇所の3行if文→1行マクロ呼び出し
- 削減: 18行→6行（12行削減）

対象箇所:
- object_fields handler trace
- instance_box handler trace
- string_box handler trace
- array_box handler trace
- map_box handler trace
- fallback(length=0) handler trace

効果:
- 保守性向上: trace条件の一元管理
- 可読性向上: 冗長なif文→簡潔なマクロ
- 一貫性向上: 全handler統一フォーマット

テスト: ビルド成功（0エラー、87警告）

src/mir/builder/if_form.rs

@@ -79,8 +79,9 @@ impl MirBuilder {
         }
         let then_value_raw = self.build_expression(then_branch)?;
         let then_exit_block = self.current_block()?;
+        let then_reaches_merge = !self.is_current_block_terminated();
         let then_var_map_end = self.variable_map.clone();
-        if !self.is_current_block_terminated() {
+        if then_reaches_merge {
             // Scope leave for then-branch
             self.hint_scope_leave(0);
             crate::mir::builder::emission::branch::emit_jump(self, merge_block)?;
@@ -116,7 +117,8 @@ impl MirBuilder {
             (void_val, None, None)
         };
         let else_exit_block = self.current_block()?;
-        if !self.is_current_block_terminated() {
+        let else_reaches_merge = !self.is_current_block_terminated();
+        if else_reaches_merge {
             // Scope leave for else-branch
             self.hint_scope_leave(0);
             self.emit_instruction(MirInstruction::Jump { target: merge_block })?;
@@ -144,8 +146,8 @@ impl MirBuilder {
         let result_val = self.normalize_if_else_phi(
             then_block,
             else_block,
-            Some(then_exit_block),
-            Some(else_exit_block),
+            if then_reaches_merge { Some(then_exit_block) } else { None },
+            if else_reaches_merge { Some(else_exit_block) } else { None },
             then_value_raw,
             else_value_raw,
             &pre_if_var_map,
@@ -176,8 +178,8 @@ impl MirBuilder {
         self.merge_modified_vars(
             then_block,
             else_block,
-            then_exit_block,
-            Some(else_exit_block),
+            if then_reaches_merge { Some(then_exit_block) } else { None },
+            if else_reaches_merge { Some(else_exit_block) } else { None },
             &pre_if_var_map,
             &then_var_map_end,
             &else_var_map_end_opt,