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/llvm_py/instructions/binop.py

@@ -125,13 +125,19 @@ def lower_binop(
         # pointer present?
         is_ptr_side = (hasattr(lhs_raw, 'type') and isinstance(lhs_raw.type, ir.PointerType)) or \
                       (hasattr(rhs_raw, 'type') and isinstance(rhs_raw.type, ir.PointerType))
-        # Explicit dst_type hint from MIR JSON?
-        force_string = False
-        try:
-            if isinstance(dst_type, dict) and dst_type.get('kind') == 'handle' and dst_type.get('box_type') == 'StringBox':
-                force_string = True
-        except Exception:
-            pass
+        # Phase 131-6 FIX: Do NOT use dst_type hint from MIR JSON!
+        # The dst_type is a forward-looking type hint that may be incorrect
+        # (e.g., "i + 1" gets StringBox hint because i is later used as string in print(i))
+        # We should only do string concat if operands are actually strings.
+        #
+        # OLD CODE (REMOVED):
+        # force_string = False
+        # try:
+        #     if isinstance(dst_type, dict) and dst_type.get('kind') == 'handle' and dst_type.get('box_type') == 'StringBox':
+        #         force_string = True
+        # except Exception:
+        #     pass
+
         # tagged string handles?（どちらかが string-ish のとき）
         any_tagged = False
         try:
@@ -143,7 +149,14 @@ def lower_binop(
                     any_tagged = (lhs in resolver.string_literals) or (rhs in resolver.string_literals)
         except Exception:
             pass
-        is_str = force_string or is_ptr_side or any_tagged
+        # Phase 131-6: Removed force_string from this check
+        is_str = is_ptr_side or any_tagged
+
+        # Phase 131-6 DEBUG
+        if os.environ.get('NYASH_BINOP_DEBUG') == '1':
+            print(f"[binop +] lhs={lhs} rhs={rhs} dst={dst}")
+            print(f"  is_ptr_side={is_ptr_side} any_tagged={any_tagged} is_str={is_str}")
+            print(f"  dst_type={dst_type}")
         if is_str:
             # Helper: convert raw or resolved value to string handle
             def to_handle(raw, val, tag: str, vid: int):