Moved common test fixtures (mock_writer, mock_reader, test_zone, player,
nearby_player, clear_state) from individual test files into a shared
conftest.py. This eliminates duplication across test_power.py, test_sleep.py,
test_combat_zaxis.py, test_quit.py, test_stamina_cues.py, and
test_stamina_cue_wiring.py.
Some test files override specific fixtures where they need custom behavior
(e.g., test_quit.py adds a close method to mock_writer, stamina tests use
smaller zones and custom player positions).
