Split stateful and raw VTXO types

[hsjoberg]

BarkVtxo currently represents two different things:

1. Wallet-owned VTXOs returned from wallet state APIs, where Bark exposes WalletVtxo.state.
2. Bare/raw VTXOs returned from APIs like sendArkoorPayment(), where Bark only returns Vtxo and no wallet state exists.

This forced NitroArk to preserve the old state: "unknown" fallback for bare VTXOs when restoring BarkVtxo.state.

We should split the types so the API models this correctly:

export type BarkRawVtxo = {
  amount: number;
  expiry_height: number;
  server_pubkey: string;
  exit_delta: number;
  anchor_point: string;
  point: string;
};

export type BarkVtxo = BarkRawVtxo & {
  state: 'Spendable' | 'Spent' | 'Locked';
};