LANTERN purge B6: delete the legacy single-ability path (sockets are THE ability model)

AbilityRef, AbilityCooldown, EffectiveAbilityStats, DefaultAbility deleted.
GoInGameServerSystem seeds the per-frame 4-socket Spark loadout UNCONDITIONALLY
(was gym-only); ClassSelectReceiveSystem + DebugOp.SetClass swap FrameId +
re-seed sockets + zero SocketCooldown; ClassSwapUtil.Apply drops newAbilityId;
EquipSystem weapons become stat-sticks (GrantedAbilityId removed from the item
blob/authoring); StatRecomputeSystem folds CharacterStatsRef + sockets only;
HUD cooldown bar reads socket 0 of SocketCooldown/EffectiveSocketStats; class
HUD readers are FrameId-only (ClassForAbility/AbilityFor deleted);
DebugModifierInjectionSystem drops CycleAbility.

390 tests green; Play-verified: a non-gym spawn gets frame=2 with Sparks
[7,8,6,9] and no console errors.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/_Project/Tests/EditMode/MetaSeedingTests.cs

@@ -31,9 +31,10 @@ namespace ProjectM.Tests
 
         static Entity MakePlayerPrefab(EntityManager em)
         {
-            var e = em.CreateEntity(typeof(LocalTransform), typeof(GhostOwner), typeof(AbilityRef), typeof(PlayerTag));
+            var e = em.CreateEntity(typeof(LocalTransform), typeof(GhostOwner), typeof(FrameId), typeof(PlayerTag));
             em.SetComponentData(e, LocalTransform.Identity);
             em.AddBuffer<StatModifier>(e);
+            em.AddBuffer<AbilitySocket>(e); // the spawn path SetBuffers the frame loadout unconditionally (LANTERN purge)
             em.AddComponent<Prefab>(e);
             return e;
         }