feat: Add DLC Input support for spliced contracts

[matthewjablack]

Overview

Implements DLC Input support within FundingInput messages to enable advanced DLC functionality like spliced contracts. This is a breaking change to the FundingInput serialization format.

BREAKING CHANGE Warning

This PR introduces breaking changes to FundingInput serialization:

• All FundingInput messages now include an optional DlcInput suffix (0x00 or 0x01 + data)
• Existing serialized FundingInput messages will fail to deserialize correctly
• New FundingInput messages are incompatible with older versions of the library
• Migration required for any stored or transmitted FundingInput data

New Features

DlcInput Message Type

• New message type: DlcInput (42773)
• Fields: localFundPubkey, remoteFundPubkey, fundValue

Enhanced FundingInput Format

• Required suffix: All FundingInputs now end with optional DlcInput data
• Format: [existing fields][0x00] or [existing fields][0x01][DlcInput data]
• Rust-DLC alignment: Matches rust-dlc optional format specification

Technical Implementation

New Serialization Format

FundingInput (UPDATED):
[inputSerialId: 8 bytes]
[prevTx: variable]
[prevTxVout: 4 bytes]
[sequence: 4 bytes]
[maxWitnessLen: 2 bytes]
[redeemScript: variable]
[0x00] // No DLC input (NEW REQUIRED BYTE)
OR
[0x01][DlcInput data] // With DLC input (NEW FORMAT)

JSON Schema Extension

{
  "inputSerialId": 1,
  "prevTx": "02000000...",
  "prevTxVout": 0,
  "sequence": 4294967295,
  "maxWitnessLen": 108,
  "redeemScript": "",
  "dlcInput": {  // NEW: Optional field
    "localFundPubkey": "023da092f6980e58d2c037173180e9a465476026ee50f96695963e8efe436f54eb",
    "remoteFundPubkey": "025476c2e83188368da1ff3e292e7acafcdb3566bb0ad253f62fc70f07aeee6357", 
    "fundValue": 100000000
  }
}

Migration Required

For Applications Using FundingInput

1. Re-serialize all stored FundingInput data using the new format
2. Update protocol versions to handle format differences

Dependencies & Tooling

• Rust-DLC alignment: Uses bennyhodl/rust-dlc@spliced-contract branch
• Test vectors: Updated dlcspecs test vectors for new format
• Migration script: Added update-dlcspecs-vectors.sh for regenerating test data

Testing

# Test new DlcInput functionality
npm test -- --grep "DlcInput"

# Test enhanced FundingInput format
npm test -- --grep "FundingInput.*DlcInput"

# Test format compatibility
npm test -- --grep "rust-dlc optional format"

# Run compatibility tests
npm test compatibility

# Regenerate test vectors
./scripts/update-dlcspecs-vectors.sh

[bennyhodl]

No comments, LGTM!