refactor(combat): replace ac_penalty with effect system

src/pf1e_simulator/combat.py

@@ -51,8 +51,8 @@ Phase 0 documented deviations from PF1e (conventions):
 - Charge: a full-round action that moves in a straight line (Bresenham) up to
   2x speed toward the closest square from which the charger can melee the
   target, then makes a single melee attack at +2. The charger takes -2 AC
-  until the start of their next turn (tracked in ``ac_penalty``, cleared in
-  ``_take_turn``). The straight line must be clear of difficult terrain,
+  until the start of their next turn (tracked as a StatModifier in
+  ``effects``, cleared in ``_take_turn``). The straight line must be clear of difficult terrain,
   obstacles, and creatures; minimum 2 cells (10 ft). ``default_policy``
   chooses a charge when the enemy is out of reach but a valid charge path
   exists.
@@ -76,9 +76,11 @@ Phase 0 documented deviations from PF1e (conventions):
 from __future__ import annotations
 
 from collections.abc import Callable
-from dataclasses import asdict, dataclass
+from dataclasses import asdict, dataclass, field
 from typing import TYPE_CHECKING
 
+from pf1e_simulator.effects import StatModifier, resolve_modifiers
+
 if TYPE_CHECKING:
     from typing import Literal
 
@@ -156,7 +158,7 @@ class CombatantState:
     side: str
     pos: Pos
     hp: int
-    ac_penalty: int = 0
+    effects: list[StatModifier] = field(default_factory=list)
     moved_this_turn: bool = False
 
     @property
@@ -250,7 +252,7 @@ class CombatEngine:
 
     def _take_turn(self, state: CombatantState) -> bool:
         """Execute one combatant's full turn; return True if the battle is over."""
-        state.ac_penalty = 0
+        state.effects.clear()
         state.moved_this_turn = False
         actions = self._policy(self, state)
         before = len(self._transcript)
@@ -358,7 +360,9 @@ class CombatEngine:
         flank = self._flanking_bonus(attacker, target) if weapon.kind == "melee" else 0
         base_bonus = bonus_override if bonus_override is not None else weapon.attack_bonus
         total = roll + base_bonus + penalty + flank
-        ac = target.combatant.ac.total - target.ac_penalty
+        ac = target.combatant.ac.total + resolve_modifiers(
+            e for e in target.effects if e.target == "ac"
+        )
         occupied = frozenset(
             s.pos for s in self._states if s.active and s is not attacker and s is not target
         )
@@ -620,7 +624,7 @@ class CombatEngine:
             )
         if not attacker.active:
             return
-        attacker.ac_penalty = _CHARGE_AC_PENALTY
+        attacker.effects.append(StatModifier(target="ac", value=-_CHARGE_AC_PENALTY))
         bonus = weapon.attack_bonus + _CHARGE_ATTACK_BONUS
         self._resolve_swing(
             attacker, target, weapon, label="charge", bonus_override=bonus

src/pf1e_simulator/effects.py

@@ -18,7 +18,7 @@ from dataclasses import dataclass
 from typing import TYPE_CHECKING, Literal
 
 if TYPE_CHECKING:
-    from collections.abc import Sequence
+    from collections.abc import Iterable
 
 BonusType = Literal[
     "",               # untyped — stacks with everything
@@ -65,7 +65,7 @@ class StatModifier:
     weapon_filter: str | None = None
 
 
-def resolve_modifiers(modifiers: Sequence[StatModifier]) -> int:
+def resolve_modifiers(modifiers: Iterable[StatModifier]) -> int:
     """Sum stat modifiers applying PF1e stacking rules.
 
     Stacking bonus types (dodge, racial, trait, untyped) all sum together.