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/classic/dbc_layouts.json

@@ -95,5 +95,14 @@
     "ID": 0, "MapID": 1, "AreaID": 2, "AreaName": 3,
     "LocLeft": 4, "LocRight": 5, "LocTop": 6, "LocBottom": 7,
     "DisplayMapID": 8, "ParentWorldMapID": 10
+  },
+  "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
   }
 }