Update `immature_transactions()` to return `HashSet<Txid>` and `spent_outpoints` rebuild to use `input_details`

[xdustinface]

Context

immature_transactions() returns Vec<Transaction> but every caller only uses .txid(), .len(), or .is_empty(). The spent_outpoints rebuild on deserialization iterates record.transaction.input — with input_details available, it can use those instead.

Parent: #68

Acceptance Criteria

• immature_transactions() returns HashSet<Txid> instead of Vec<Transaction>
• All callers updated (dash-spv tests, key-wallet tests, wallet_checker tests)
• spent_outpoints rebuild on deserialization uses record.input_details (OutPoint from transaction.input[detail.index].previous_output) instead of iterating all tx inputs
• Tests pass, cargo clippy clean

Notes

• Depends on Add InputDetail, OutputDetail, TransactionDirection to TransactionRecord #69 (input_details must exist in TransactionRecord)
• immature_transactions() in wallet_info_interface.rs finds immature coinbase UTXOs and collects their txids — the Transaction was never needed
• spent_outpoints rebuild in managed_account/mod.rs deserialization: currently record.transaction.input.iter().map(|i| i.previous_output) — change to record.input_details.iter().map(|d| record.transaction.input[d.index].previous_output)
• Files: key-wallet/src/wallet/managed_wallet_info/wallet_info_interface.rs, key-wallet/src/managed_account/mod.rs, dash-spv/tests/dashd_sync/setup.rs, key-wallet/tests/spv_integration_tests.rs