feat: use contract_flags bit 0 to redirect refund to accepter

[matthewjablack]

Summary

Use bit 0 of the existing contract_flags byte on DlcOffer to signal that all refund transaction
proceeds go to the accepter. When the flag is set, the offerer's refund output is Amount::ZERO
(dust-discarded) and the accepter receives total_collateral.

Changes

• Add REFUND_TO_ACCEPTER_FLAG constant (0x01) in dlc/src/lib.rs
• Add contract_flags: u8 parameter to create_dlc_transactions, create_spliced_dlc_transactions, and
  create_cets_and_refund_tx
• Modify refund output construction to redirect all funds to accepter when flag is set
• Add contract_flags: u8 field to OfferedContract struct with serde default for backwards compatibility
• Wire contract_flags through OfferDlc ↔ OfferedContract conversions
• Thread contract_flags through channel transaction creation (create_channel_transactions,
  create_renewal_channel_transactions)
• Propagate contract_flags from offered_contract at all call sites in contract_updater.rs and
  channel_updater.rs
• Add contract_flags to OfferedContract binary serialization
• Propagate contract_flags from offered_contract at all call sites in contract_updater.rs and
  channel_updater.rs
• Add contract_flags to OfferedContract binary serialization
• Add unit test verifying single-output refund TX pays total collateral to accepter's payout SPK
• Update existing tests, benchmarks, and compatibility tests with default 0 flag

create_renewal_channel_transactions)

• Propagate contract_flags from offered_contract at all call sites in contract_updater.rs and
  channel_updater.rs
• Add contract_flags to OfferedContract binary serialization
• Add unit test verifying single-output refund TX pays total collateral to accepter's payout SPK
• Update existing tests, benchmarks, and compatibility tests with default 0 flag