feat: USDCx outbound — burn via BridgeUserAgreement + release tracking

[sadiq1971]

Goal

USDCx outbound bridging (Canton USDCx -> Ethereum USDC): burn on Canton via BridgeUserAgreement_Burn, Circle releases USDC on Ethereum (minus fee); relayer tracks the release.

Mechanics (confirmed, July 2026)

• BridgeUserAgreement_Burn takes destination domain (0 = Ethereum), amount (max 6 dp), Ethereum recipient address, a UUID request id, and the holding contract ids to burn.
• It is a user-party choice: custodial users -> middleware signs directly; external-key users -> prepare/execute signing flow (same pattern as pkg/transfer).
• Factory + disclosed contracts from the DA Utilities burn-mint-factory registry endpoint.
• Circle validates the burn and releases USDC to the Ethereum recipient; timing is Circle-controlled; a Circle withdrawal fee applies.

Flow

1. Dapp calls POST /api/v2/bridge/withdraw/prepare (external key) or the custodial variant; api-server builds BridgeUserAgreement_Burn.
2. execute submits the burn and registers the transfer with the relayer (bridge_key=xreserve, stage="", metadata: burn_request_id).
3. xreserve adapter Step: "" -> awaiting_release — watch Ethereum for the xReserve release matching burn_request_id -> completed with the release tx hash. The adapter never submits anything; it only observes Circle.

Work items

• Burn command builder (choice args + disclosed contracts via burn-mint-factory endpoint)
• Prepare/execute wiring for external keys; direct exercise for custodial
• Holding selection for the burn amount (UTXO-style holdings; reuse existing holding-selection logic if applicable)
• Transfer registration on execute (burn_request_id in metadata)
• bridges/xreserve withdrawal Step: release watcher on the xReserve contract
• Fee surfacing: GET /api/v2/bridge/tokens and the withdraw response show Circle's fee and "timing controlled by Circle"
• Devstack: stub release (scripted ERC-20 transfer from a fake xReserve) so e2e covers the stage machine
• e2e: burn (custodial + external-key) -> release stub -> transfer completed

Depends on

• xreserve inbound adapter (shares circle/ledger clients, config, onboarding)
• Bridge API (prepare/execute endpoints, registration)