Bind spork signatures to network

[NateIsern]

Motivation

• A cross-network replay/authentication flaw was possible because spork signatures only covered nSporkID, nValue, and nTimeSigned while mainnet and testnet shared the same SporkKey.
• Signatures were not bound to any chain-specific context such as the network ID, MessageStart bytes, or genesis hash, so a signed spork for one network could verify on another.
• A minimal change is required to add domain separation to spork signatures and prevent cross-network replay while keeping the signing/verification flow intact.

Description

• Add a helper GetSporkSignatureMessage(const CSporkMessage&) that composes a domain-separated signing string including Params().NetworkIDString(), Params().MessageStart() (hex), Params().HashGenesisBlock(), and the delimited spork fields.
• Update CSporkManager::CheckSignature and CSporkManager::Sign to use GetSporkSignatureMessage so both signing and verification operate on the same network-bound payload.
• Preserve the existing calls to the compact-signature helpers (obfuscationSigner.SignMessage / VerifyMessage) so the change is limited to the signed message construction.

Testing

• Ran git diff --check which completed successfully with no issues.
• Attempted to run ./autogen.sh but the command failed in this environment due to a missing aclocal, so a full autotools build/test could not be executed here.

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Codex Task