fix: XRP Issue 40-byte encoding, nil opts guards, public ComputeSigna…

[panterose]

Context

We are building an invoice factoring platform on XRPL using XLS-65 (Single Asset Vault) and XLS-66 (Lending Protocol) together. The workflow involves:

1. Creating Vaults (XLS-65) to hold LP liquidity
2. Creating LoanBrokers that draw from these Vaults
3. Creating Loans (XLS-66) that require dual-signing between lender and borrower

Issues Encountered

VaultCreate failing with `invalidTransaction`:

• Root cause: The `Issue` type was encoding XRP as only 20 bytes (currency), but the decoder expected 40 bytes (currency + issuer zeros)
• This caused a `parser out of bounds` error during transaction serialization

LoanSet dual-signing not possible:

• The `computeSignature` method in `wallet.go` was private
• XLS-66 LoanSet requires both lender and borrower to sign the same transaction, needing external access to signature computation

Nil pointer dereference:

• Calling `SubmitTx` or `SubmitTxAndWait` with nil options caused a panic

Type of change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Refactoring

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code where needed
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective
• New and existing unit tests pass locally with my changes

Changes

File	Fix
`issue.go`	XRP Issues now return 40 bytes (currency + 20 zero bytes for issuer)
`client.go`	Added nil guards for `opts` parameter
`wallet.go`	Added public `ComputeSignature` wrapper for dual-signing

Testing

• All binary-codec tests pass
• tokinvoice/shared/xrpl builds and tests pass
• VaultCreate and LoanSet transactions now serialize correctly