refactor(joinir): Pattern 3 ExitMeta化 - Hardcoded ValueIds削除

Refactoring 5.1: Pattern 3 を Pattern 4 と同じ ExitMeta ベースアーキテクチャに統一化

Changes:
1. loop_with_if_phi_minimal.rs
   - 署名: Option<JoinModule> → Result<(JoinModule, JoinFragmentMeta), String>
   - ExitMeta 動的生成ロジック追加（sum, count）
   - インポート追加: carrier_info::{ExitMeta, JoinFragmentMeta}

2. pattern3_with_if_phi.rs
   - Hardcoded 定数削除（PATTERN3_K_EXIT_*_ID 2個削除）
   - Manual exit binding 42行 → ExitMetaCollector 4行に置き換え
   - インポート追加: ExitMetaCollector

3. loop_patterns/with_if_phi.rs
   - Result型変更に対応（.ok()? で変換）

Benefits:
- Pattern 3/4 アーキテクチャ統一化 ✅
- 19行純削減（+55 -74行、3ファイル合計） ✅
- Hardcoded ValueIds 完全撤廃 ✅
- Phase 213 AST-based generalization の基盤強化 ✅

Tests: All tests passing, loop_if_phi.hako outputs "sum=9" correctly

src/mir/join_ir/lowering/loop_patterns/with_if_phi.rs

@@ -110,7 +110,8 @@ pub fn lower_loop_with_conditional_phi_to_joinir(
     let mut join_value_space = JoinValueSpace::new();
 
     // Generate JoinIR module
-    let _join_module = lower_loop_with_if_phi_pattern(placeholder_scope, &mut join_value_space)?;
+    // Phase 213: Updated to handle Result<(JoinModule, JoinFragmentMeta), String>
+    let _result = lower_loop_with_if_phi_pattern(placeholder_scope, &mut join_value_space).ok()?;
 
     // Phase 188-Impl-3: Pattern 3 is now integrated via the router
     // This function delegates to loop_with_if_phi_minimal which generates JoinModule