docs: ドキュメント配置ルール（SSOT）確立

## 追加内容
- CLAUDE.md にドキュメント配置ルール（SSOT）セクション追加
- DOCS_LAYOUT.md (SSOT): 置き場所ルール定義
- phases/README.md: Phase ドキュメント説明
- design/README.md: 設計図ドキュメント説明
- investigations/README.md: 調査ログ説明

## ルール概要
1. **Phase 文書** → phases/phase-<N>/
2. **設計図** → design/
3. **調査ログ** → investigations/ （結論を 10-Now/20-Decisions に反映）

## 導線
- CLAUDE.md で概要説明
- DOCS_LAYOUT.md で詳細定義（SSOT）
- 各フォルダ README で参照方法

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

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

src/llvm_py/instructions/binop.py

@@ -138,18 +138,25 @@ def lower_binop(
         # except Exception:
         #     pass
 
-        # tagged string handles?（どちらかが string-ish のとき）
+        # Phase 196: TypeFacts SSOT - Only check for actual string types (not use-site demands)
+        # Check if BOTH operands are known to be strings from their definition
         any_tagged = False
         try:
             if resolver is not None:
-                if hasattr(resolver, 'is_stringish'):
-                    any_tagged = resolver.is_stringish(lhs) or resolver.is_stringish(rhs)
-                # literal strings are tracked separately
-                if not any_tagged and hasattr(resolver, 'string_literals'):
+                # Only check string_literals (TypeFacts), NOT is_stringish (TypeDemands)
+                if hasattr(resolver, 'string_literals'):
                     any_tagged = (lhs in resolver.string_literals) or (rhs in resolver.string_literals)
+                # Check if resolver has explicit type information (MirType::String or StringBox)
+                if not any_tagged and hasattr(resolver, 'value_types'):
+                    lhs_ty = resolver.value_types.get(lhs)
+                    rhs_ty = resolver.value_types.get(rhs)
+                    lhs_str = lhs_ty and (lhs_ty.get('kind') == 'string' or
+                                         (lhs_ty.get('kind') == 'handle' and lhs_ty.get('box_type') == 'StringBox'))
+                    rhs_str = rhs_ty and (rhs_ty.get('kind') == 'string' or
+                                         (rhs_ty.get('kind') == 'handle' and rhs_ty.get('box_type') == 'StringBox'))
+                    any_tagged = lhs_str or rhs_str
         except Exception:
             pass
-        # Phase 131-6: Removed force_string from this check
         is_str = is_ptr_side or any_tagged
 
         # Phase 131-6 DEBUG
@@ -184,12 +191,25 @@ def lower_binop(
                     return val
                 return ir.Constant(i64, 0)
 
-            # Decide route: handle+handle when both sides are string-ish; otherwise pointer+int route.
+            # Phase 196: TypeFacts SSOT - Use handle+handle only when BOTH are strings
             lhs_tag = False; rhs_tag = False
             try:
-                if resolver is not None and hasattr(resolver, 'is_stringish'):
-                    lhs_tag = resolver.is_stringish(lhs)
-                    rhs_tag = resolver.is_stringish(rhs)
+                if resolver is not None:
+                    # Check string_literals (actual string constants)
+                    if hasattr(resolver, 'string_literals'):
+                        lhs_tag = lhs in resolver.string_literals
+                        rhs_tag = rhs in resolver.string_literals
+                    # Check value_types for String/StringBox types
+                    if not lhs_tag and hasattr(resolver, 'value_types'):
+                        lhs_ty = resolver.value_types.get(lhs)
+                        if lhs_ty and (lhs_ty.get('kind') == 'string' or
+                                      (lhs_ty.get('kind') == 'handle' and lhs_ty.get('box_type') == 'StringBox')):
+                            lhs_tag = True
+                    if not rhs_tag and hasattr(resolver, 'value_types'):
+                        rhs_ty = resolver.value_types.get(rhs)
+                        if rhs_ty and (rhs_ty.get('kind') == 'string' or
+                                      (rhs_ty.get('kind') == 'handle' and rhs_ty.get('box_type') == 'StringBox')):
+                            rhs_tag = True
             except Exception:
                 pass
             if lhs_tag and rhs_tag: