enr, node: v4 ENR signature verify and signed records

[ch4r10t33r]

Summary

• Decode: accept 64-byte (r ‖ s, EIP-778 v4) or 65-byte (r ‖ s ‖ v) signature strings on the wire.
• verifyV4RecordPayload / verifyV4RecordSignature: Keccak-256 over RLP([seq, k₁, v₁, …]), then ECDSA (secp256k1) verify against the record secp256k1 key; require id = v4.
• encodeV4RecordSigned: build a valid v4 record RLP from secret key, seq, and pairs payload (used for tests and node handshake ENR).
• Node: handleHandshake verifies the ENR cryptographic signature before continuing; buildMinimalEnrRlp now signs with self.secret_key (outbound handshake ENR is no longer a zero placeholder).

Tests

• Official devp2p enr.md example enr: URI (signature + node id).
• Sign/verify roundtrip with a synthetic key.

Addresses #20 (remaining: optional omitted handshake record when peer already knows the same seq).