* core/stateless: fix witness RLP backward compat with WIT/0 peers

Nodes upgraded from WIT/0 to WIT/1 were incorrectly jailing WIT/0 peers
with "rlp: too few elements for stateless.ExtWitness". A 5-field
ExtWitness encoding (Context, Headers, Codes, State, Keys) was
accidentally introduced during an upstream geth merge, replacing the
original 3-field wire format (Context, Headers, State) that WIT/0 peers
still use.

Introduce BorWitness as the canonical 3-field wire format. EncodeRLP now
encodes using BorWitness (State only, codes excluded). DecodeRLP tries
BorWitness first and falls back to ExtWitness for backward compatibility
with peers that temporarily ran the extended encoding.

ExtWitness is retained as the representation for the JSON debug API
(ExecutionWitness, ExecutionWitnessByHash) and as the fallback decode
target for the transitional period.

* Add test