feat(rules): scale healing spells with caster level

PF1e (SRD spells/cure-light-wounds.md): 'cures 1d8 points of damage +1
point per caster level (maximum +5)'; (spells/cure-moderate-wounds.md):
'cures 2d8 points of damage +1 point per caster level (maximum +10)'.
A HealEffect now carries the same DamageFlatScaling spec as damage
effects (S1b) and resolves it through effective_formula(caster_level)
at cast time, so the flat bonus grows with the caster and stops at the
stated cap. The engine passes the caster's level into _apply_heal_effect
(required keyword: no silent level-0 under-heal); Maximize Spell
maximizes the scaled formula. Cure Light/Moderate Wounds ship the SRD
values; inflict_light_wounds already carried its scaling_flat.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

tests/test_spells.py

@@ -588,7 +588,9 @@ class TestCureLightWounds:
                                          target_side="players")
         t.hp = 10
         engine._cast_spell(c, t, "Cure Light Wounds")
-        assert t.hp == 10 + (5 + 1)
+        # Caster level 5: flat bonus = min(5, (5 // 1) * 1) = 5 (SRD
+        # cure-light-wounds.md: "+1 point per caster level (maximum +5)").
+        assert t.hp == 10 + (5 + 5)
 
     def test_heal_capped_at_max(self) -> None:
         caster = _make_caster()