fix(rules): allow a Will save to halve channeled energy damage

PF1e CRB (Channel Energy): creatures damaged by channeled energy get a Will
save (DC 10 + half the channeler's level + her Charisma modifier) for half
damage (floor). Harm mode only; heal mode unchanged. Charisma is used RAW
even for casters whose spellcasting ability differs (channeling is a
Charisma class feature).

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

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

tests/test_class_abilities.py

@@ -155,15 +155,32 @@ def test_oni_channel_harm_does_not_damage_living_enemy() -> None:
 
 
 def test_oni_channel_harm_damages_undead() -> None:
-    """Harm mode damages undead creatures in the burst regardless of side."""
+    """Harm mode damages undead creatures in the burst regardless of side;
+    a failed Will save takes full damage (CRB Channel Energy)."""
     oni = _combatant("oni", resources={"channel_energy": 6}, hp_max=30)
     lich = _combatant("lich", hp_max=100, creature_type="undead")
-    engine, me, enemy = _engine(oni, [lich], rng_queue=[11])
+    engine, me, enemy = _engine(oni, [lich], rng_queue=[11, 5])
 
     action = Action(kind="channel_energy", target_id=lich.id, channel_mode="harm")
     engine._execute(me, action)
 
     assert enemy[0].hp == 100 - (1 + 11)  # 2d6 = 1 + roll_die(11)
+    # Will save fails: d20 5 + will 4 = 9 < DC 14 (10 + lvl//2 2 + cha 2)
+    assert any("Will DC14" in line and "FAILED full" in line for line in engine._transcript)
+
+
+def test_oni_channel_harm_will_save_halves_damage() -> None:
+    """CRB Channel Energy: an undead that passes the Will save takes half
+    damage (floor)."""
+    oni = _combatant("oni", resources={"channel_energy": 6}, hp_max=30)
+    lich = _combatant("lich", hp_max=100, creature_type="undead")
+    engine, me, enemy = _engine(oni, [lich], rng_queue=[11, 20])
+
+    action = Action(kind="channel_energy", target_id=lich.id, channel_mode="harm")
+    engine._execute(me, action)
+
+    assert enemy[0].hp == 100 - (1 + 11) // 2  # 2d6 = 12, saved for half = 6
+    assert any("Will DC14" in line and "SAVED half" in line for line in engine._transcript)
 
 
 def test_oni_channel_heal_heals_living_enemies_in_range() -> None: