feat: add shared witness encoding helpers

[abelmega]

Summary

Add shared witness encoding helpers to stateless-common and switch RpcClient witness decoding to use the shared implementation.

What Changed

• add stateless-common::witness_encoding for the shared witness RPC wire format
• expose helpers for:
  • compressing witness payloads
  • decompressing witness payloads
  • encoding versioned witness RPC responses
  • decoding versioned witness RPC responses
• migrate the fetch_witness_raw decode path in rpc_client.rs to call the shared decoder
• replace the remaining hard-coded v0: test stub in rpc_client.rs with the shared version prefix constant
• add round-trip and error-path tests for the new witness encoding module
• tighten module comments to clearly distinguish response prefix vs payload

Notes

• the witness RPC wire format remains unchanged: v0:base64(zstd(level=9, bincode-legacy((SaltWitness, MptWitness))))
• rpc_client.rs request flow and retry behavior are unchanged
• this PR does not yet migrate witness response encoding call sites outside the new helper module

Validation

• cargo check -p stateless-common
• cargo test -p stateless-common witness_encoding

[claude]

The PR currently has no labels, but it appears the enhancement label is missing.

The title (feat: add shared witness encoding helpers) and the diff confirm this is a new feature: it introduces the witness_encoding module with shared helpers for compressing/decompressing/encoding/decoding witness RPC payloads, migrates rpc_client.rs to use the shared decoder, and adds round-trip tests. This matches the enhancement label description ("New feature or request").

[claude]

Clean extraction. The layering is clear (compress/decompress for raw bytes, encode/decode for the full versioned string), error types are typed, and the rpc_client.rs migration is straightforward.

Two things before merge — see inline comments:

1. Missing error-path tests for the Base64 and decompression branches in decode_witness_response.
2. nit: WitnessEncodingError::Serialize uses #[source] while Compress uses #[from] — inconsistency without a clear reason.

[claude]

LGTM. Previous feedback addressed — #[from] added to WitnessEncodingError::Serialize and all three error-path tests added. The module is well-structured and the encoding contract is clearly documented.