feat(combat): AoO and concentration on casting

src/pf1e_simulator/combat.py

@@ -640,6 +640,49 @@ class CombatEngine:
                 return True
         return False
 
+    def _check_casting_aoo(self, caster: CombatantState, spell_level: int) -> bool:
+        """Check for AoO from casting a spell in a threatened square.
+
+        Returns True if the caster dropped or failed concentration (spell fizzles).
+        """
+        for enemy in self._enemies_threatening(caster, caster.pos):
+            if enemy.combatant.id in self._aoo_used:
+                continue
+            hp_before = caster.hp
+            self._resolve_aoo(enemy, caster)
+            if not caster.active:
+                self._log(
+                    f"round {self._current_round} {caster.combatant.id}: "
+                    f"casting interrupted by AoO from {enemy.combatant.id}"
+                )
+                return True
+            # Concentration check if damaged
+            dmg = hp_before - caster.hp
+            if dmg > 0 and not self._check_concentration(caster, spell_level, dmg):
+                self._log(
+                    f"round {self._current_round} {caster.combatant.id}: "
+                    f"concentration check failed (DC {15 + 2 * spell_level}), spell fizzles"
+                )
+                return True
+        return False
+
+    def _check_concentration(
+        self, caster: CombatantState, spell_level: int, damage_taken: int
+    ) -> bool:
+        """Concentration check: DC = 15 + 2 * spell_level + damage_taken.
+
+        Combat Casting feat gives +4 bonus.
+        """
+        dc = 15 + 2 * spell_level + damage_taken
+        has_combat_casting = any(f.name == "Combat Casting" for f in caster.combatant.features)
+        bonus = 4 if has_combat_casting else 0
+        roll = self._rng.d20()
+        # Use caster level + ability mod for concentration check
+        caster_level = caster.combatant.level
+        # For concentration, the check is d20 + caster_level + ability_mod (usually Con)
+        con_mod = (caster.combatant.abilities.con_score - 10) // 2
+        total = roll + caster_level + con_mod + bonus
+        return total >= dc
     def can_step5_to_attack(self, state: CombatantState, target: CombatantState) -> bool:
         """True if a 5-foot step toward ``target`` would put ``state`` in weapon range."""
         path = self._move_path(state, target)
@@ -878,6 +921,10 @@ class CombatEngine:
                 f"round {self._current_round} {caster.combatant.id}: "
                 f"cast {spell_name} -> NO LINE OF EFFECT"
             )
+            return
+        # Check for AoO from casting in threatened square (before resolving spell)
+        if self._check_casting_aoo(caster, spell.level):
+            return
         # DC = 10 + spell level + caster's casting ability modifier
         ability_name = caster.combatant.casting_ability.lower()
         ability_score = getattr(caster.combatant.abilities, f"{ability_name}_score")