fix: use str() instead of .value for realm i18n display (#98)

* fix: use str() instead of .value for realm i18n display

Fix bug where user-facing messages displayed raw enum values like
"FOUNDATION_ESTABLISHMENT" instead of translated names like "筑基".

The Realm and Stage classes already have __str__ methods that return
i18n translated text, but several places were incorrectly using
.value which returns the raw enum string.

Changed files:
- src/classes/single_choice.py: item exchange messages
- src/classes/kill_and_grab.py: loot messages
- src/classes/fortune.py: fortune discovery messages
- src/classes/avatar/inventory_mixin.py: purchase error messages

Also added unit tests and integration tests to prevent regression.

* test: add integration tests for all modified files

Add tests covering:
- kill_and_grab.py: context string realm display
- fortune.py: weapon/auxiliary intro realm display
- inventory_mixin.py: can_buy_item error message realm display

src/classes/avatar/inventory_mixin.py

@@ -190,7 +190,8 @@ class InventoryMixin:
         if isinstance(obj, Elixir):
             # 境界限制
             if obj.realm > self.cultivation_progress.realm:
-                return False, f"境界不足，无法承受药力 ({obj.realm.value})"
+                # 使用 str() 来触发 Realm 的 __str__ 方法进行 i18n 翻译。
+                return False, f"境界不足，无法承受药力 ({str(obj.realm)})"
 
             # 耐药性/生效中检查
             for consumed in self.elixirs: