FIX: detect external SP spends to prevent phantom balances

[abhayguptas]

Problem

HDSilentPaymentsWallet.fetchUtxo() does not detect when a Silent Payment (SP) UTXO is spent externally.

The wallet refreshes regular UTXOs through Electrum, but does not query the tweaked Taproot addresses belonging to its SP UTXOs.

As a result, an SP UTXO can remain marked as unspent after being spent from another device or wallet. This can cause the wallet to show an incorrect balance and attempt to spend an already-spent output.

Fix

During fetchUtxo(), query the tweaked Taproot addresses of currently unspent SP UTXOs using the existing Electrum interface.

• Keep isSpent as false when the outpoint is present.
• Set isSpent to true when the request succeeds but the outpoint is missing.
• Keep the existing isSpent state unchanged if the Electrum request fails.
• Keep spent SP UTXOs in _utxo so transaction history is preserved.

Tests

Added deterministic regression tests covering:

• externally spent SP UTXOs
• still-unspent SP UTXOs
• Electrum/network failure
• multiple SP UTXOs
• preserving spent SP UTXOs for transaction history
• existing parent fetchUtxo() behavior

Verification

• TypeScript compilation: passed
• ESLint: passed
• Focused tests: 6/6 passed
• Full unit test suite: 274/274 passed

fixes #158