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.
