Add settled promise for send/melt flows

[Egge21M]

Summary

Expose a “wait until settled” promise on Send and Melt flows so callers can await terminal completion without wiring event listeners or polling.

Problem

• SendApi.executePreparedSend() (packages/core/api/SendApi.ts) returns a pending operation + token, but callers must manually watch events or call checkPendingOperation() to know when it finalizes or rolls back.
• QuotesApi.executeMelt() may return a pending operation; callers must poll checkPendingMelt() or listen for melt-op:* events to know when it settles.
• This is especially painful for CLI/short-lived apps that want to “do the thing then exit”.

Proposal

Return an additional promise from the public APIs that resolves when the flow reaches a terminal state.
Suggested API shape (additive)

• Send
  • SendApi.executePreparedSend() → { operation, token, settled }
• Melt
  • QuotesApi.executeMelt() / executeMeltByQuote() → { operation, settled }
settled resolves with the terminal operation, and rejects on explicit failure or timeout.
    Settlement definition
• Send: send:finalized or send:rolled-back
• Melt: melt-op:finalized or melt-op:rolled-back (and failed if exposed)

Implementation notes

• Build a small helper that:
  • Checks current state first (fast-path if already terminal).
  • Subscribes to EventBus events (CoreEvents) and auto-unsubscribes on resolve/reject.
  • Optionally accepts timeout (default off or reasonable).
• Ensure listeners are cleaned up to avoid leaks.
• Document that settlement depends on watchers/processors being enabled (or add an internal polling fallback if desired).

Acceptance criteria

• Send and Melt flows expose a settled promise without breaking existing return types.
• settled resolves on terminal state transitions and unsubscribes listeners.
• Immediate resolution if the operation is already terminal.
• Tests cover:
  • immediate terminal resolve
  • pending → terminal resolve
  • rollback/failure handling
  • unsubscribe cleanup

Notes

• Mint flow should be tackled once MintSaga is implemented.

[ye0man]

@Egge21M does #128 close this out or still relevant?