feat: implement single-funded DLC transactions

[matthewjablack]

Summary

Implements full support for single-funded DLC transactions where one party provides all inputs while the other party contributes zero inputs but still participates in the contract.

Changes Made

Single-Funded DLC Support

• Added dust output filtering in CreateFundTransaction and CreateBatchFundTransaction
• Fixed fee computation for single-funded DLCs where funding party pays all fees
• Updated change calculation to properly handle zero-input parties
• Corrected serial ID handling to only include non-dust outputs in vout calculations

Fee Distribution Logic

• Funding party pays all fees including the non-funding party's CET fees
• Zero-input party returns zero fees in GetChangeOutputAndFees functions
• Proper change recalculation accounting for both parties' collateral and all fees

Transaction Structure Improvements

• Dust filtering removes zero-value change outputs from funding transactions
• Supports no-change transactions for enhanced privacy
• Maintains correct output ordering with serial ID sorting

Comprehensive Testing

• CreateSingleFundedDlcTransactions - Basic single-funded DLC test
• CreateBatchSingleFundedDlcTransactions - Batch single-funded DLC test
• CreateSingleFundedDlcTransactionsNoChange - No-change single-funded DLC test
• Fixed existing test bug in CreateCetTransactionNotEnoughInputTest

Benefits

• Enables asymmetric DLC funding for real-world use cases
• Reduces on-chain footprint by filtering dust outputs
• Supports privacy-focused transactions with no change outputs
• Maintains backward compatibility with existing dual-funded DLCs

Use Cases

• Asymmetric funding scenarios where one party provides all capital
• Privacy-enhanced DLCs with minimal on-chain footprint
• Simplified DLC workflows for certain business models

All tests pass and the implementation maintains full compatibility with existing DLC functionality.