feat(mir): Phase 21.7 Step 3 - Methodization実装完了

🎯 Global("BoxName.method/arity") → Method{receiver=singleton} 変換

実装内容:
1. builder.rs: static_box_singletons フィールド追加
   - BoxName → ValueId マッピングでシングルトン管理
2. unified_emitter.rs: methodization ロジック実装
   - HAKO_MIR_BUILDER_METHODIZE=1 で有効化
   - decode_static_method() でstatic box method判定
   - singleton instance を NewBox で生成（キャッシュ済み）
   - Callee::Global → Callee::Method 変換

動作確認:
- デフォルト(OFF): call_global Calculator.add/2 (既存挙動維持)
- トグル(ON): new Calculator → call %singleton.add (methodization)
- RC=0 両モード動作確認済み

テスト:
- apps/tests/phase217_methodize_test.hako 追加
- [methodize] Global(...) → Method{...} トレース出力

環境変数:
- HAKO_MIR_BUILDER_METHODIZE=1: methodization 有効化
- NYASH_METHODIZE_TRACE=1: 変換ログ出力

Phase 21.7 Step 3 完了！🎊
次: ドキュメント更新

apps/tests/phase217_methodize_test.hako

@@ -0,0 +1,15 @@
+// Phase 21.7: Methodization test
+// Tests Global("BoxName.method/arity") → Method{receiver=singleton} conversion
+
+static box Calculator {
+    add(a, b) {
+        return a + b
+    }
+
+    main() {
+        // This will be called as Global("Calculator.add/2") by default
+        // With HAKO_MIR_BUILDER_METHODIZE=1, it becomes Method{Calculator.add, recv=singleton}
+        print("Result: " + ("" + Calculator.add(10, 20)))
+        return 0
+    }
+}

src/mir/builder.rs

@@ -221,6 +221,12 @@ pub struct MirBuilder {
     /// - Some(true): App mode (static box Main.main is entry)
     /// - Some(false): Script/Test mode (top-level Program runs sequentially)
     pub(super) root_is_app_mode: Option<bool>,
+
+    /// 🎯 Phase 21.7: Static box singleton instances for methodization
+    /// Maps BoxName → ValueId of singleton instance (created on demand)
+    /// Used when HAKO_MIR_BUILDER_METHODIZE=1 to convert Global("BoxName.method/arity")
+    /// to Method{receiver=singleton} calls
+    pub(super) static_box_singletons: HashMap<String, ValueId>,
 }
 
 impl MirBuilder {
@@ -282,6 +288,7 @@ impl MirBuilder {
             in_unified_boxcall_fallback: false,
             recursion_depth: 0,
             root_is_app_mode: None,
+            static_box_singletons: HashMap::new(), // Phase 21.7: methodization support
         }
     }
 

src/mir/builder/calls/unified_emitter.rs

@@ -173,6 +173,57 @@ impl UnifiedCallEmitterBox {
             }
         };
 
+        // 🎯 Phase 21.7: Methodization (HAKO_MIR_BUILDER_METHODIZE=1)
+        // Convert Global("BoxName.method/arity") → Method{receiver=static singleton}
+        if std::env::var("HAKO_MIR_BUILDER_METHODIZE").ok().as_deref() == Some("1") {
+            if let Callee::Global(ref name) = callee {
+                let name_clone = name.clone(); // Clone to avoid borrow checker issues
+                // Try to decode as static box method
+                if let Some((box_name, method, arity)) = crate::mir::naming::decode_static_method(&name_clone) {
+                    // Check if arity matches provided args
+                    if arity == args.len() {
+                        // Get or create static box singleton instance
+                        let singleton = if let Some(&existing) = builder.static_box_singletons.get(box_name) {
+                            existing
+                        } else {
+                            // Create new singleton instance
+                            let singleton_id = builder.next_value_id();
+                            builder.emit_instruction(MirInstruction::NewBox {
+                                dst: singleton_id,
+                                box_type: box_name.to_string(),
+                                args: Vec::new(), // Static box singleton, no constructor args
+                            })?;
+                            // Register type information
+                            builder.value_types.insert(
+                                singleton_id,
+                                crate::mir::MirType::Box(box_name.to_string()),
+                            );
+                            builder.value_origin_newbox.insert(singleton_id, box_name.to_string());
+                            // Cache for future use
+                            builder.static_box_singletons.insert(box_name.to_string(), singleton_id);
+                            singleton_id
+                        };
+
+                        // Convert to Method call
+                        callee = Callee::Method {
+                            box_name: box_name.to_string(),
+                            method: method.to_string(),
+                            receiver: Some(singleton),
+                            certainty: crate::mir::definitions::call_unified::TypeCertainty::Known,
+                            box_kind: crate::mir::definitions::call_unified::CalleeBoxKind::StaticCompiler,
+                        };
+
+                        if std::env::var("NYASH_METHODIZE_TRACE").ok().as_deref() == Some("1") {
+                            eprintln!(
+                                "[methodize] Global({}) → Method{{{}.{}, recv=%{}}}",
+                                name_clone, box_name, method, singleton.0
+                            );
+                        }
+                    }
+                }
+            }
+        }
+
         // Structural guard FIRST: prevent static compiler boxes from being called with runtime receivers
         // 箱理論: CalleeGuardBox による構造的分離
         // (Guard may convert Method → Global, so we check BEFORE materializing receiver)