feat(skills): end a dead caster's skill when a client reports it (CasterDead)

When a client gets EchoStartSkill (or SyncSkill) from a caster it sees
as dead, aimed at another object, it logs "msgEchoStartSkill
msgCasterDead" and sends CasterDead (120: optional i64Caster, optional
uiSkillHandle) through that target
(LWOSkillComponent::msgEchoStartSkill 0x00d5dc90). 303 live packets, the
target nearly always the attacked player, the caster a spawned enemy;
the next messages were mostly Die and SetStunned for the player. DLU
dropped it.

The server now ends that skill on the caster: its behaviors with that
handle are dropped (end entries run, pending timers and syncs discarded,
its projectiles removed), so a hit an enemy scheduled before dying does
not land later. It only does so when the server also sees the caster as
dead, so a client can't cancel a living enemy's attack. What live did
with the message is inferred from the name and when it was sent.

Check in game: kill an enemy in the middle of a slow or charged attack
(e.g. a Maelstrom horseman or a spider queen add): its attack does not
hit after it has died, and other enemies' attacks still hit normally.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dGame/dGameMessages/GameMessageHandler.cpp

@@ -169,6 +169,7 @@ namespace {
 		{ SELECT_SKILL, []() { return std::make_unique<SelectSkill>(); } },
 		{ START_SKILL, []() { return std::make_unique<StartSkill>(); } },
 		{ SYNC_SKILL, []() { return std::make_unique<SyncSkill>(); } },
+		{ CASTER_DEAD, []() { return std::make_unique<CasterDead>(); } },
 		{ REQUEST_SERVER_PROJECTILE_IMPACT, []() { return std::make_unique<RequestServerProjectileImpact>(); } },
 
 		// Combat