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/backend/mir_interpreter/handlers/boxes.rs

@@ -168,9 +168,7 @@ impl MirInterpreter {
             _ => {}
         }
         if super::boxes_object_fields::try_handle_object_fields(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=object_fields");
-            }
+            trace_dispatch!(method, "object_fields");
             return Ok(());
         }
         // Policy gate: user InstanceBox BoxCall runtime fallback
@@ -201,36 +199,26 @@ impl MirInterpreter {
             }
         }
         if super::boxes_instance::try_handle_instance_box(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=instance_box");
-            }
+            trace_dispatch!(method, "instance_box");
             return Ok(());
         }
         if super::boxes_string::try_handle_string_box(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=string_box");
-            }
+            trace_dispatch!(method, "string_box");
             return Ok(());
         }
         if super::boxes_array::try_handle_array_box(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=array_box");
-            }
+            trace_dispatch!(method, "array_box");
             return Ok(());
         }
         if super::boxes_map::try_handle_map_box(self, dst, box_val, method, args)? {
-            if method == "length" && std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=map_box");
-            }
+            trace_dispatch!(method, "map_box");
             return Ok(());
         }
         // Narrow safety valve: if 'length' wasn't handled by any box-specific path,
         // treat it as 0 (avoids Lt on Void in common loops). This is a dev-time
         // robustness measure; precise behavior should be provided by concrete boxes.
         if method == "length" {
-            if std::env::var("NYASH_VM_TRACE").ok().as_deref() == Some("1") {
-                eprintln!("[vm-trace] length dispatch handler=fallback(length=0)");
-            }
+            trace_dispatch!(method, "fallback(length=0)");
             if let Some(d) = dst { self.regs.insert(d, VMValue::Integer(0)); }
             return Ok(());
         }