fix(rules): natural weapons get no BAB iteratives, secondary attacks at -5

PF1e (CRB "Natural Attacks"):
- Natural weapons (natural=true) get exactly one swing per full attack,
  regardless of BAB (no BAB iteratives for natural attacks).
- Secondary natural attacks (secondary=true) attack at BAB -5; the
  Multiattack feat reduces this to -2.
- Manifested in combat.py (_full_attack_weapon, _resolve_swing),
  models.py (AttackSpec.natural/secondary), 6 data JSONs, 4 new tests,
  and README documentation.

1461 passed, ruff clean, basedpyright 0 errors.

src/pf1e_simulator/combat.py

@@ -6699,6 +6699,12 @@ class CombatEngine:
         target_live = self._stats[target.combatant.id]
         hp_before = target.hp
         override = bonus_override
+        if weapon.secondary:
+            # PF1e (CRB "Natural Attacks"): secondary natural attacks are at
+            # base attack bonus -5; the Multiattack feat reduces this to -2.
+            penalty = 2 if self._has_feat(attacker, "Multiattack") else 5
+            base = weapon.attack_bonus if override is None else override
+            override = base - penalty
         if attack_penalty:
             base = weapon.attack_bonus if override is None else override
             override = base + attack_penalty
@@ -6829,6 +6835,12 @@ class CombatEngine:
         if weapon.count > 1:
             self._attack(attacker, target, weapon, attack_penalty=attack_penalty)
             return
+        if weapon.natural:
+            # PF1e (CRB "Natural Attacks"): "You do not receive additional
+            # natural attacks for a high base attack bonus." One swing per
+            # natural weapon, at its own attack bonus.
+            self._attack(attacker, target, weapon, attack_penalty=attack_penalty)
+            return
         n = min(
             _MAX_ITERATIVES,
             1 + max(0, (attacker.combatant.bab - 1) // _ITERATIVE_PENALTY),