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/abilities.py

@@ -139,5 +139,11 @@ DODGE = AbilitySpec(
     ),
 )
 
-# Skill Focus: +3 to a skill (not yet modeled — skills deferred)
-# Defined as a placeholder for future expansion.
+# Power Attack: active feat — declare at turn start, -X atk / +2X dmg on all
+# melee attacks this turn, where X = BAB//4 + 1 (min 1). No passive modifiers;
+# the engine checks for this feat and the ``power_attack`` flag on state.
+POWER_ATTACK = AbilitySpec(
+    name="Power Attack",
+    category="feat",
+    source_detail="feat lv1",
+)