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).
When combat begins, any active power-up task on either the attacker
or defender should be cancelled to prevent background power changes
during combat. This ensures players can't continue charging while
fighting.
The fix checks both entities for a _power_task attribute and cancels
it if present, then clears the reference.
Implements power level management system with tick-based power-up loop.
Players can raise PL toward max_pl (costs stamina per tick), lower PL
instantly, set exact PL targets, and cancel ongoing power-ups.
