fix: honor element custom codecs for uint8 slices

[Solaris-star]

Summary

Named uint8 element types that implement CustomEncoder/CustomDecoder were still packed as msgpack bin when used in a slice:

type Role uint8
func (r Role) EncodeMsgpack(enc *msgpack.Encoder) error { return enc.EncodeString("BACKEND") }

msgpack.Marshal(Role(1))      // string ✅
msgpack.Marshal([]Role{1, 2}) // bin blob ❌

Fix

In encoder/decoder type dispatch, only take the []byte shortcut when the element type has no custom codec (value or pointer receiver).

Test plan

• go test .
• TestByteKindSliceHonorsElementCustomCodec

Fixes #391