Fix: Correct ProRegTx extraPayload parsing and serialization (BLS signature handling)

[LexxXell]

📝 Summary

This PR fixes the incorrect parsing and serialization logic for ProRegTx extra payloads (DIP-3 / DIP-23), which caused deserialization errors and txid mismatches when re-encoding transactions using the latest Dash Core ≥ v19/v20 formats.

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🔍 Problem

• For payloadVersion ≥ 2 (DIP-23 “basic” BLS scheme), the payloadSig field was being serialized as varbytes (CompactSize + bytes).

• According to the current Dash Core implementation, v2+ payloads store the signature as a fixed-length 90 or 96 byte sequence without CompactSize prefix.

• As a result:

  • deserialize() could fail with “attempt to read past end of buffer”;
  • txid() validation failed (expected … got …) because re-serialized bytes differed by one byte (missing CompactSize fix).

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🛠️ Changes

• Updated DashProRegTx.read_vds() and serialize() to follow DIP-3 / DIP-23 spec:

  • Read/write payloadSig as raw 90 / 96 bytes for version ≥ 2 (no length prefix).
  • Keep legacy read_varbytes() behavior for version < 2.
  • Compute signature length as remaining bytes in extraPayload when deserializing.
  • Added explicit sanity checks for 90 / 96 bytes.
  • Preserve correct combined header version (type << 16 | base_version) to ensure txid stability.

• Minor cleanup: consistent big-endian port serialization, added asserts for field sizes.

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✅ Verification

• Tested against known ProRegTx v2 samples from mainnet/testnet:

  • Deserialization succeeds without “read past end” errors.
  • Re-serialization yields identical hex and matching txid.

• Backward-compatible with legacy (v1) payloads — varbytes signature still supported.

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🧩 References

• [DIP-0003 – Deterministic Masternode Lists](https://github.com/dashpay/dips/blob/master/dip-0003.md)
• [DIP-0023 – BLS Basic Scheme Migration](https://github.com/dashpay/dips/blob/master/dip-0023.md)
• [Dash Core Docs: Special Transactions / ProRegTx](https://docs.dash.org/en/stable/docs/core/reference/transactions-special-transactions.html)

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⚙️ Impact

This fix ensures that ProRegTx parsing and re-serialization are binary-accurate to Dash Core ≥ v19+, preventing signature truncation and txid mismatches in wallet and node implementations.