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/monsters/giant_belostomatid.json

@@ -1,46 +1,70 @@
 {
-  "name": "Giant Belostomatid",
-  "level": 3,
-  "size": "Large",
-  "cr": "3",
-  "xp": 800,
-  "source": "Bestiary > Giant Belostomatid",
-  "notes": "Digestive enzyme (DC 15 Fort or paralyzed 1d4 rounds + 1d2 Con damage/round) and grab on claws not modeled in Phase 0; Int - (mindless) modeled as 0; immune to mind-affecting not modeled; swim 40 ft / fly 20 ft (poor) not modeled, land speed only.",
-  "abilities": {
-    "str_score": 15,
-    "dex_score": 13,
-    "con_score": 16,
-    "int_score": 0,
-    "wis_score": 11,
-    "cha_score": 2
-  },
-  "hp_max": 30,
-  "ac": { "total": 15, "touch": 10, "flat_footed": 14 },
-  "bab": 3,
-  "initiative_mod": 1,
-  "speed_land_ft": 20,
-  "saves": { "fort": 7, "ref": 2, "will": 1 },
-  "attacks": [
+ "name": "Giant Belostomatid",
+ "level": 3,
+ "size": "Large",
+ "cr": "3",
+ "xp": 800,
+ "source": "Bestiary > Giant Belostomatid",
+ "notes": "Digestive enzyme (DC 15 Fort or paralyzed 1d4 rounds + 1d2 Con damage/round) and grab on claws not modeled in Phase 0; Int - (mindless) modeled as 0; immune to mind-affecting not modeled; swim 40 ft / fly 20 ft (poor) not modeled, land speed only.",
+ "abilities": {
+  "str_score": 15,
+  "dex_score": 13,
+  "con_score": 16,
+  "int_score": 0,
+  "wis_score": 11,
+  "cha_score": 2
+ },
+ "hp_max": 30,
+ "ac": {
+  "total": 15,
+  "touch": 10,
+  "flat_footed": 14
+ },
+ "bab": 3,
+ "initiative_mod": 1,
+ "speed_land_ft": 20,
+ "saves": {
+  "fort": 7,
+  "ref": 2,
+  "will": 1
+ },
+ "attacks": [
+  {
+   "name": "bite",
+   "kind": "melee",
+   "attack_bonus": 4,
+   "damage": [
     {
-      "name": "bite",
-      "kind": "melee",
-      "attack_bonus": 4,
-      "damage": [{ "formula": "1d6", "types": ["piercing"] }],
-      "damage_bonus": 2,
-      "crit_mult": 2,
-      "reach_ft": 10
-    },
-    {
-      "name": "claw",
-      "kind": "melee",
-      "attack_bonus": 4,
-      "damage": [{ "formula": "1d4", "types": ["slashing"] }],
-      "damage_bonus": 2,
-      "crit_mult": 2,
-      "count": 2,
-      "reach_ft": 10
+     "formula": "1d6",
+     "types": [
+      "piercing"
+     ]
     }
-  ],
-  "cmb": 6,
-  "cmd": 17
-}
+   ],
+   "damage_bonus": 2,
+   "crit_mult": 2,
+   "reach_ft": 10,
+   "natural": true
+  },
+  {
+   "name": "claw",
+   "kind": "melee",
+   "attack_bonus": 4,
+   "damage": [
+    {
+     "formula": "1d4",
+     "types": [
+      "slashing"
+     ]
+    }
+   ],
+   "damage_bonus": 2,
+   "crit_mult": 2,
+   "count": 2,
+   "reach_ft": 10,
+   "natural": true
+  }
+ ],
+ "cmb": 6,
+ "cmd": 17
+}