fix(core): charge sole funder full base weights in single-funded DLCs

[matthewjablack]

Summary

DualFundingTxFinalizer always split the fund tx and CET base weights in half per party, so in a single-funded DLC (counterparty has no inputs) nobody paid the other half. This matches ddk-dlc, which bills the sole funder the full base weights, and fixes coin selection undershooting the funds ddk requires (a 5000 sat contract at 10 sat/vB was accepting 7950 sats of inputs when 8000 were needed).

Changes

• TxFinalizer.ts: when the counterparty has zero inputs, the funding party pays the full fund tx base weight and the full 500-weight CET base instead of half. Dual-funded fees are unchanged (107 / 249 per party).
• CoinSelect.ts: dualFees and dualFundingCoinSelect take an optional trailing singleFunded flag (default false) that drops the synthetic accept input so the threshold is sized correctly.
• Tests pin the single-funder numbers (1530 fund + 1470 CET = 3000 at 10 sat/vB with one P2WPKH input), the symmetric accepter case, unchanged dual-funded fees, and the 7950 + 3000 coin selection case.
• BatchDlcTxBuilder.calculateMaxCollateral already passes empty accept inputs, so it picks up the fix with no changes.

Note: upstream rust-dlc always splits the base weight in half. ddk-dlc is the one that deviates, but it is the counterparty in practice and the transaction underpays otherwise.