fix(rules): cure spells damage undead instead of healing

SRD Cure Light Wounds: 'Since undead are powered by negative energy,
this spell deals damage to them instead of curing their wounds. An
undead creature can apply spell resistance, and can attempt a Will save
to take half damage.'

- _apply_heal_effect takes the rolled save; an undead target takes the
  rolled healing as damage (halved on a successful save; SR already
  resolved upstream, DR never applies to spell energy, no bleeding stop
  or stabilization). Living targets are bit-identical.
- cure_light/cure_moderate descriptions fixed: the SRD save line is
  'Will half (harmless)', not 'negates'.
- Deafened 20% verbal spell failure DEFERRED to the README « Limites
  hors perimetre »: SpellSpec carries no components data (would need a
  verbal flag on all 108 spells) and sound-based Perception is not
  modeled either.

data/spells/cure_light_wounds.json

@@ -13,5 +13,5 @@
       "scaling_flat": { "amount": 1, "per_levels": 1, "max": 5 }
     }
   ],
-  "description": "Cures 1d8 points of damage +1 point per caster level (maximum +5). Will save negates (harmless)."
+  "description": "Cures 1d8 points of damage +1 point per caster level (maximum +5). Will half (harmless); since undead are powered by negative energy, the spell deals damage to them instead (Will half, SRD cure light wounds)."
 }