fix(combat): correct spell DC formula

tests/test_spells.py

@@ -56,6 +56,7 @@ def _make_caster(cid: str = "caster", *, level: int = 5) -> Combatant:
         speed_land_ft=30,
         saves=Saves(fort=3, ref=4, will=4),
         attacks=[attack],
+        casting_ability="int",
     )
 
 
@@ -366,18 +367,18 @@ class TestSpellDispatchViaExecute:
         )
         assert t.hp < hp_before
 
-
 class TestSpellDcScalesWithLevel:
-    """Given: a caster with level N
-    When: a spell with base_dc 10 is cast
-    Then: the DC is 10 + N."""
+    """Given: a caster with level N and casting ability mod M
+    When: a spell of level L is cast
+    Then: the DC is 10 + L + M."""
 
-    def test_dc_is_base_plus_level(self) -> None:
-        caster = _make_caster(level=5)
+    def test_dc_is_ten_plus_spell_level_plus_ability_mod(self) -> None:
+        caster = _make_caster(level=5)  # int 18 -> mod +4
         target = _make_target()
         engine, c, t = _make_engine(caster, target, [20, 1, 1, 1, 1, 1, 1])
-        engine._cast_spell(c, t, "Fireball")
-        assert any("DC15" in line for line in engine._transcript)
+        engine._cast_spell(c, t, "Fireball")  # fireball is level 3
+        # DC = 10 + spell_level(3) + int_mod(4) = 17
+        assert any("DC17" in line for line in engine._transcript)
 
 
 class TestSpellDamageWithDr: