Phase 123 proper完了：hako_check JoinIR実装（環境変数選択可能化）

## 実装内容

### 1. 環境変数フラグ追加
- NYASH_HAKO_CHECK_JOINIR でJoinIR/Legacy経路を切り替え可能
- src/config/env/hako_check.rs で hako_check_joinir_enabled() 実装
- デフォルト: false（レガシー経路）で後方互換性確保

### 2. MIR Builder JoinIR スイッチ
- cf_if() メソッドにフラグチェック追加
- try_cf_if_joinir() プレースホルダー実装（Phase 124で完全実装）
- JoinIR → legacy フォールバック機構を構築

### 3. テストケース作成（4個）
- phase123_simple_if.hako
- phase123_nested_if.hako
- phase123_while_loop.hako
- phase123_if_in_loop.hako

### 4. テスト結果
✅ Legacy path: 4/4 PASS
✅ JoinIR path: 4/4 PASS
（JoinIR path は現在フォールバック経由で動作）

### 5. ドキュメント更新
- environment-variables.md: NYASH_HAKO_CHECK_JOINIR 記載
- phase121_hako_check_joinir_design.md: Phase 123実装セクション追加
- hako_check_design.md: 2パス実行フロー図を追加
- CURRENT_TASK.md: Phase 123完了を記録

## 数値成果

- 新規ファイル: 2個 (config/env/hako_check.rs, test cases × 4, test script)
- 修正ファイル: 6個
- 総追加行数: 335行
- ビルド: Zero errors

## 設計・実装の特徴

✅ Environment variable で簡単に経路切り替え可能
✅ レガシー経路を完全に保持（後方互換性）
✅ JoinIR基盤を Phase 124 での完全実装に向けて構築
✅ フォールバック機構でリスク最小化

## 次のステップ

Phase 124: JoinIR 完全実装＆デフォルト化
- try_cf_if_joinir() を IfSelectLowerer と統合
- Loop JoinIR 統合追加
- JoinIR をデフォルト経路に変更
- NYASH_LEGACY_PHI=1 で legacy フォールバック可能に

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/builder/control_flow.rs

@@ -10,16 +10,83 @@ impl super::MirBuilder {
     }
 
     /// Control-flow: if
+    ///
+    /// # Phase 123: hako_check JoinIR Integration
+    ///
+    /// When NYASH_HAKO_CHECK_JOINIR=1 is set, if statements will be routed through
+    /// JoinIR lowering path instead of the legacy PHI generation path.
+    ///
+    /// Default: legacy path (for backward compatibility)
+    /// Phase 124: JoinIR will become the default
     pub(super) fn cf_if(
         &mut self,
         condition: ASTNode,
         then_branch: ASTNode,
         else_branch: Option<ASTNode>,
     ) -> Result<ValueId, String> {
+        // Phase 123: Check hako_check JoinIR flag
+        let use_joinir = crate::config::env::hako_check_joinir_enabled();
+
+        if use_joinir {
+            let debug = std::env::var("NYASH_HAKO_CHECK_JOINIR_DEBUG").is_ok();
+            if debug {
+                eprintln!("[cf_if/joinir] Routing if statement through JoinIR lowering");
+            }
+
+            // Phase 123: For now, try JoinIR path and fallback to legacy if needed
+            // Phase 124 will make this strict (no fallback)
+            match self.try_cf_if_joinir(&condition, &then_branch, &else_branch, debug) {
+                Ok(Some(value)) => {
+                    if debug {
+                        eprintln!("[cf_if/joinir] Successfully lowered through JoinIR");
+                    }
+                    return Ok(value);
+                }
+                Ok(None) => {
+                    if debug {
+                        eprintln!("[cf_if/joinir] JoinIR not applicable, falling back to legacy");
+                    }
+                }
+                Err(e) => {
+                    if debug {
+                        eprintln!("[cf_if/joinir] JoinIR error: {}, falling back to legacy", e);
+                    }
+                }
+            }
+        }
+
         // current impl is a simple forward to canonical lowering
         self.lower_if_form(condition, then_branch, else_branch)
     }
 
+    /// Phase 123: Try JoinIR lowering for if statements
+    ///
+    /// Returns:
+    /// - Ok(Some(value)): Successfully lowered through JoinIR
+    /// - Ok(None): JoinIR not applicable (pattern not recognized)
+    /// - Err(e): JoinIR error
+    fn try_cf_if_joinir(
+        &mut self,
+        _condition: &ASTNode,
+        _then_branch: &ASTNode,
+        _else_branch: &Option<ASTNode>,
+        _debug: bool,
+    ) -> Result<Option<ValueId>, String> {
+        // Phase 123: Placeholder for JoinIR if lowering
+        // The actual JoinIR lowering requires a complete function to work on,
+        // so we need to build the MIR first and then apply lowering.
+        // For Phase 123, we'll return None to fallback to legacy path.
+        // Phase 124 will implement the full JoinIR integration.
+
+        // TODO Phase 124: Implement actual JoinIR if lowering here
+        // This will require:
+        // 1. Build if statement with legacy path
+        // 2. Apply JoinIR IfSelectLowerer to the resulting MIR
+        // 3. Return the lowered result
+
+        Ok(None)
+    }
+
     /// Control-flow: loop
     ///
     /// # Phase 49: JoinIR Frontend Mainline Integration