Summary

Adds an authenticated SSZ-REST transport for the Engine API alongside the existing JSON-RPC path.

• Adds /engine/v*/payloads, /engine/v*/forkchoice, /engine/v*/blobs, /engine/v1/client/version, and /engine/v1/capabilities handlers.
• Advertises SSZ endpoint capabilities through engine_exchangeCapabilities while preserving JSON-RPC fallback.
• Implements Erigon-native SSZ wire types using cl/ssz encode/decode patterns.
• Adds focused SSZ-REST handler/wire tests.
• Adds Kurtosis devnet material for Prysm + Erigon + Spamoor with 4s slots.

Verification

• Agent verifier completed all checks under /root/gevm-erigon/agi/sszrest_verifier.
• Live Kurtosis interop run completed with Prysm + Erigon + Spamoor.
• Devnet used 4 second slots (seconds_per_slot = 4, slot_duration_ms = 4000).
• Erigon logs showed SSZ-REST getPayload, newPayload, and forkchoice traffic.
• Prysm synced blocks and issued forkchoice updates.
• Spamoor confirmed transactions.
• Final verifier scan found no fatal Engine API errors.