phase15: implement Phase S root treatment for break control flow

🔧 **Phase S (Immediate Stabilization) Implementation**
- Create control flow utilities module (src/mir/utils/)
- Refactor loop_builder.rs duplicated code to utilities
- Fix PHI incoming predecessor capture per ChatGPT Pro analysis

📊 **AI Collaborative Analysis Complete**
- Task agent: Root cause identification
- Gemini: Strategic 3-phase approach
- codex: Advanced type inference solution (archived)
- ChatGPT Pro: Definitive staged treatment strategy

🗂️ **Documentation & Archive**
- Strategy document: docs/development/strategies/break-control-flow-strategy.md
- codex solutions: archive/codex-solutions/ (100+ lines changes)
- Update CLAUDE.md with 2025-09-23 progress

⚡ **Expected Impact**
- Resolve collect_prints null return issue
- Eliminate code duplication (4 locations unified)
- Foundation for Phase M (PHI unification) and Phase L (BuildOutcome)

🎯 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/utils/mod.rs

@@ -0,0 +1,19 @@
+/*!
+ * MIR Utilities - Phase 15 段階的根治戦略の共通ユーティリティ
+ * 
+ * フェーズS: 即効止血
+ * フェーズM: PHI一本化  
+ * フェーズL: 根本解決
+ * 
+ * 全フェーズで使用する汎用関数を提供
+ */
+
+pub mod control_flow;
+
+// 外部公開API
+pub use control_flow::{
+    is_current_block_terminated,
+    capture_actual_predecessor_and_jump,
+    collect_phi_incoming_if_reachable,
+    execute_statement_with_termination_check,
+};