feat: implement SMSG_PLAY_SPELL_VISUAL with SpellVisual DBC chain lookup

Parse SMSG_PLAY_SPELL_VISUAL (casterGuid + visualId) and spawn a
transient M2 spell effect at the caster's world position.

DBC chain: SpellVisual.dbc → SpellVisualKit.dbc → SpellVisualEffectName.dbc
Lookup priority: CastKit.SpecialEffect0, fallback to MissileModel.
Models are lazy-loaded and cached by path; instances auto-expire after 3.5s.
DBC layouts added to all four expansion layout files (Classic/TBC/WotLK/Turtle).

Data/expansions/wotlk/dbc_layouts.json

@@ -116,5 +116,14 @@
   },
   "LFGDungeons": {
     "ID": 0, "Name": 1
+  },
+  "SpellVisual": {
+    "ID": 0, "CastKit": 2, "ImpactKit": 3, "MissileModel": 8
+  },
+  "SpellVisualKit": {
+    "ID": 0, "BaseEffect": 5, "SpecialEffect0": 11, "SpecialEffect1": 12, "SpecialEffect2": 13
+  },
+  "SpellVisualEffectName": {
+    "ID": 0, "FilePath": 2
   }
 }