test: add shared world fixtures

[snoopuppy582]

Fixes #24

Summary

• Added shared small_world and loaded_world pytest fixtures in tests/conftest.py.
• Refactored world tests to use the shared fixtures instead of a local helper.
• Added a small default-config smoke assertion through loaded_world.

Tests

• uv run --python 3.12 --extra dev pytest tests/test_world.py -q
• uv run --python 3.12 --extra dev pytest -q
• uv run --python 3.12 --extra dev ruff check .
• git diff --check

Notes

• Kept the change limited to shared test fixtures and the existing world tests.