fix(llvm): Phase 131-6 調査 - TAG-RUN 3バグ発見（1修正/1部分/1未修正）

Phase 131-6: Infinite Loop Bug 調査完了

発見したバグ（3件）:
1. Bug #1: Copy-to-PHI 命令（SSA 違反）✅ 修正完了
   - instruction_rewriter.rs: PHI destination への Copy をスキップ
2. Bug #2: Type Inference 混同（String vs Integer）⚠️ 部分修正
   - binop.py: force_string ロジック削除
3. Bug #3: SSA Dominance Violation ❌ 未修正
   - MIR builder が定義前に値を使用（根本問題）

変更ファイル:
- src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs:
  - Lines 428-443: header PHI への Copy スキップ追加
- src/llvm_py/instructions/binop.py:
  - Lines 128-159: force_string 削除、Phase 131-6 コメント追加
- docs/development/current/main/phase131-3-llvm-lowering-inventory.md:
  - 3バグの詳細追記

テスト結果:
- Case A/B2: ✅ 退行なし
- Case B: ❌ infinite loop 継続（Bug #3 が原因）
- Simple Add: ❌ 0 を出力（期待: 1）

Next: Phase 131-6 続き - MIR SSA dominance 根治

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs

@@ -417,12 +417,31 @@ pub(super) fn merge_and_rewrite(
                             }
                         } else {
                             // Insert Copy instructions for parameter binding
+                            // Phase 131-6 FIX: Skip Copy when dst is a header PHI destination
+                            // The PHI node will receive the value as an incoming edge instead
                             for (i, arg_val_remapped) in args.iter().enumerate() {
                                 if i < target_params.len() {
                                     let param_val_original = target_params[i];
                                     if let Some(param_val_remapped) =
                                         remapper.get_value(param_val_original)
                                     {
+                                        // Phase 131-6: Check if this would overwrite a header PHI dst
+                                        let is_header_phi_dst = loop_header_phi_info
+                                            .carrier_phis
+                                            .values()
+                                            .any(|entry| entry.phi_dst == param_val_remapped);
+
+                                        if is_header_phi_dst {
+                                            if debug {
+                                                eprintln!(
+                                                    "[cf_loop/joinir] Phase 131-6: Skip param binding to PHI dst {:?} (PHI receives value via incoming edge)",
+                                                    param_val_remapped
+                                                );
+                                            }
+                                            // Skip - PHI will receive this value as incoming edge
+                                            continue;
+                                        }
+
                                         new_block.instructions.push(MirInstruction::Copy {
                                             dst: param_val_remapped,
                                             src: *arg_val_remapped,