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).
Players must be at same altitude (both flying or both grounded) to initiate combat. Attack fails with 'You can't reach them from here!' if altitude differs.
