Transaction Prepare

Build unsigned XDR to store a VC in Vault.

POST /tx/prepare/store

• Body: { owner: string; vcId: string; didUri: string; fields: Record<string, unknown>; vaultContractId?: string; issuer?: string }

• Returns: { unsignedXdr: string }

• Validation errors: owner_required, vcId_required, didUri_required, bad_request (simulation)

• Source: api/testnet/src/controllers/txPrepareStoreController.ts:5-45