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.

data/pcs/ierlieth.json

@@ -1,59 +1,62 @@
 {
-  "name": "Ierlieth",
-  "level": 4,
-  "size": "Tiny",
-  "abilities": {
-    "str_score": 8,
-    "dex_score": 17,
-    "con_score": 11,
-    "int_score": 10,
-    "wis_score": 10,
-    "cha_score": 9
-  },
-  "hp_max": 29,
-  "ac": {
-    "total": 17,
-    "touch": 15,
-    "flat_footed": 14
-  },
-  "bab": 3,
-  "initiative_mod": 3,
-  "speed_land_ft": 15,
-  "speed_fly_ft": 40,
-  "saves": {
-    "fort": 3,
-    "ref": 6,
-    "will": 3
-  },
-  "attacks": [
+ "name": "Ierlieth",
+ "level": 4,
+ "size": "Tiny",
+ "abilities": {
+  "str_score": 8,
+  "dex_score": 17,
+  "con_score": 11,
+  "int_score": 10,
+  "wis_score": 10,
+  "cha_score": 9
+ },
+ "hp_max": 29,
+ "ac": {
+  "total": 17,
+  "touch": 15,
+  "flat_footed": 14
+ },
+ "bab": 3,
+ "initiative_mod": 3,
+ "speed_land_ft": 15,
+ "speed_fly_ft": 40,
+ "saves": {
+  "fort": 3,
+  "ref": 6,
+  "will": 3
+ },
+ "attacks": [
+  {
+   "id": "ierlieth:bite",
+   "name": "Bite",
+   "kind": "melee",
+   "attack_bonus": 4,
+   "damage": [
     {
-      "id": "ierlieth:bite",
-      "name": "Bite",
-      "kind": "melee",
-      "attack_bonus": 4,
-      "damage": [
-        {
-          "formula": "1d4",
-          "types": ["piercing"]
-        }
-      ],
-      "damage_bonus": 1,
-      "crit_range": 20,
-      "crit_mult": 2,
-      "reach_ft": 5,
-      "count": 1
+     "formula": "1d4",
+     "types": [
+      "piercing"
+     ]
     }
-  ],
-  "source": "fiches_personnages/ierlieth_sheet.json",
-  "features": [
-    "Drake (race)",
-    "Toughness",
-    "Power Attack"
-  ],
-  "spells": [],
-  "casting_ability": "wis",
-  "spell_slots": {},
-  "resources": {},
-  "metamagic_feats": [],
-  "skills": {}
+   ],
+   "damage_bonus": 1,
+   "crit_range": 20,
+   "crit_mult": 2,
+   "reach_ft": 5,
+   "count": 1,
+   "natural": true
+  }
+ ],
+ "source": "fiches_personnages/ierlieth_sheet.json",
+ "features": [
+  "Drake (race)",
+  "Toughness",
+  "Power Attack"
+ ],
+ "spells": [],
+ "casting_ability": "wis",
+ "spell_slots": {},
+ "resources": {},
+ "metamagic_feats": [],
+ "skills": {}
 }