* 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
