fix(rules): shield of faith grants its SRD deflection AC buff (+2, +1 per six caster levels, max +5) via a new BuffEffect flat scaling

tests/test_spells.py

@@ -636,6 +636,35 @@ class TestBullsStrength:
         assert any("buff" in line for line in engine._transcript)
 
 
+class TestShieldOfFaithScaling:
+    """Given: SRD shield-of-faith "+2 deflection bonus to AC, +1 per six
+    caster levels (maximum +5)"
+    When: casters of different levels cast it
+    Then: the AC modifier is 2 + min(3, level // 6) — +2 at 5th, +3 at 6th,
+    +5 at 18th."""
+
+    def _ac_value(self, level: int) -> int:
+        caster = _make_caster(level=level)
+        target = _make_target()
+        engine, c, t = _make_engine(caster, target, [3, 3], target_pos=(1, 0),
+                                         target_side="players")
+        engine._cast_spell(c, t, "Shield of Faith")
+        ac_mods = [m for b in t.active_buffs for m in b.modifiers if m.target == "ac"]
+        assert len(ac_mods) == 1
+        assert ac_mods[0].bonus_type == "deflection"
+        return ac_mods[0].value
+
+    def test_base_bonus_below_sixth_level(self) -> None:
+        assert self._ac_value(5) == 2
+
+    def test_first_step_at_sixth_level(self) -> None:
+        assert self._ac_value(6) == 3
+
+    def test_capped_at_five(self) -> None:
+        assert self._ac_value(18) == 5
+        assert self._ac_value(20) == 5
+
+
 class TestBlessAlliesOnly:
     """Given: a caster casting Bless (SRD: 50-ft burst, "the caster and all
     allies")