fix(host): make C# item custom values gob-safe

[MymaQc]

Summary

This fixes a panic when C# plugins set custom item values that are later encoded by Dragonfly through encoding/gob.

C# item custom values are transported through ValuesNBT. When those values contain compounds or byte arrays, they may be decoded as map[string]any or fixed byte arrays before being passed to item.Stack.WithValue. Dragonfly later serializes stack values with gob, which requires concrete interface types to be registered and byte arrays to use a gob-safe representation.

Changes

• Register supported custom value container types for gob.
• Normalize fixed byte arrays from decoded NBT into []byte before storing them on item stacks.
• Add a regression test covering byte arrays, nested compounds, and gob encoding of custom item values.