feat(abilities): add Power Attack active feat (-X atk / +2X dmg, melee only)

Power Attack is the first active feat: a free-action toggle declared at
turn start. When state.power_attack is True and the combatant has the
POWER_ATTACK feature, melee attacks apply -X to the attack roll and
+2X to damage, where X = BAB//4 + 1 (min 1). Ranged attacks are
unaffected. The flag is cleared at the start of each turn via
_take_turn, like other per-turn state.

- abilities.py: POWER_ATTACK constant (AbilitySpec, no passive effects)
- combat.py: _has_feat helper, _power_attack_amt, resolve_attack
  applies penalty to attack total + crit confirm, bonus to damage
- tests/test_power_attack.py: 14 tests (amount at BAB 1/4/8/12, flag
  false, feat missing, attack penalty, damage bonus, ranged unaffected,
  flag cleared on turn)
- pyproject.toml: add SLF001 + RUF059 to test per-file-ignores (white-box
  testing accesses private helpers, partial tuple unpacking)
- README.md: document Power Attack in abilities.py section, 307 tests

src/pf1e_simulator/combat.py

@@ -162,6 +162,7 @@ class CombatantState:
     effects: list[StatModifier] = field(default_factory=list)
     conditions: list[Condition] = field(default_factory=list)
     moved_this_turn: bool = False
+    power_attack: bool = False
 
     @property
     def active(self) -> bool:
@@ -266,6 +267,7 @@ class CombatEngine:
         """Execute one combatant's full turn; return True if the battle is over."""
         state.effects.clear()
         state.moved_this_turn = False
+        state.power_attack = False
         actions = self._policy(self, state)
         before = len(self._transcript)
         for action in actions:
@@ -378,6 +380,16 @@ class CombatEngine:
                 mods.append(m)
         return mods
 
+    def _has_feat(self, state: CombatantState, feat_name: str) -> bool:
+        """True if the combatant has a feature with the given name."""
+        return any(f.name == feat_name for f in state.combatant.features)
+
+    def _power_attack_amt(self, attacker: CombatantState) -> int:
+        """Power Attack exchange amount: X = BAB//4 + 1 (min 1). Returns 0 if inactive."""
+        if not attacker.power_attack or not self._has_feat(attacker, "Power Attack"):
+            return 0
+        return max(1, attacker.combatant.bab // 4 + 1)
+
     def resolve_attack(
         self,
         attacker: CombatantState,
@@ -394,7 +406,8 @@ class CombatEngine:
         base_bonus = bonus_override if bonus_override is not None else weapon.attack_bonus
         atk_mods = self._stat_modifiers(attacker, "attack", weapon_name=weapon.name)
         attack_mod = resolve_modifiers(atk_mods)
-        total = roll + base_bonus + penalty + flank + attack_mod
+        pa_amt = self._power_attack_amt(attacker) if weapon.kind == "melee" else 0
+        total = roll + base_bonus + penalty + flank + attack_mod - pa_amt
         ac = target.combatant.ac.total + resolve_modifiers(self._stat_modifiers(target, "ac"))
         occupied = frozenset(
             s.pos for s in self._states if s.active and s is not attacker and s is not target
@@ -413,11 +426,12 @@ class CombatEngine:
         if hit:
             if roll != _NATURAL_ONE and roll >= weapon.crit_range:
                 confirm = self._rng.d20()
-                confirm_total = confirm + base_bonus + penalty + flank + attack_mod
+                confirm_total = confirm + base_bonus + penalty + flank + attack_mod - pa_amt
                 crit = confirm != _NATURAL_ONE and confirm_total >= ac
             dmg_mods = self._stat_modifiers(attacker, "damage", weapon_name=weapon.name)
             damage = sum(c.formula.roll(self._rng) for c in weapon.damage) + weapon.damage_bonus
             damage += resolve_modifiers(dmg_mods)
+            damage += pa_amt * 2 if weapon.kind == "melee" else 0
             if crit:
                 damage *= weapon.crit_mult
             damage = self._apply_dr(damage, weapon, target)