fix(combat): correct concentration DCs by situation

src/pf1e_simulator/combat.py

@@ -748,21 +748,41 @@ class CombatEngine:
             # Concentration check if damaged
             dmg = hp_before - caster.hp
             if dmg > 0 and not self._check_concentration(caster, spell_level, dmg):
+                dc = self._concentration_dc(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"
+                    f"concentration check failed (DC {dc}), spell fizzles"
                 )
                 return True
         return False
 
+    def _concentration_dc(
+        self, caster: CombatantState, spell_level: int, damage_taken: int
+    ) -> int:
+        """Concentration DC for the caster's current situation.
+
+        - injured while casting: => 10 + damage_taken + spell_level
+        - grappled or pinned:    => 10 + grappler's CMB + spell_level
+        - casting defensively:   => 15 + 2 * spell_level
+        """
+        if caster.grappled_with is not None:
+            grappler = next(
+                (s for s in self._states if s.combatant.id == caster.grappled_with), None
+            )
+            cmb = self._maneuver_cmb(grappler) if grappler is not None else 0
+            return 10 + cmb + spell_level
+        if damage_taken > 0:
+            return 10 + damage_taken + spell_level
+        return 15 + 2 * spell_level
+
     def _check_concentration(
         self, caster: CombatantState, spell_level: int, damage_taken: int
     ) -> bool:
-        """Concentration check: DC = 15 + 2 * spell_level + damage_taken.
+        """Concentration check: DC depends on situation.
 
-        Combat Casting feat gives +4 bonus.
+        Roll = d20 + caster_level + casting ability mod (+4 Combat Casting).
         """
-        dc = 15 + 2 * spell_level + damage_taken
+        dc = self._concentration_dc(caster, 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()