feat(wallet): UTXO pool management for high-frequency transactions

[sgbett]

Summary

• Add UTXOPool class with acquire/release/status for instant output consumption from pre-provisioned baskets
• Add ReplenishmentWorker background thread: scavenges right-sized outputs before splitting larger ones
• Add update_output_basket to StorageAdapter for zero-cost basket reassignment
• Wire utxo_pool factory into WalletClient with optional auto_replenish
• 45 new specs covering lifecycle, concurrency (barrier pattern), scavenge/split, error handling

Test plan

• All 1316 wallet specs pass (0 failures)
• RuboCop clean on all changed files
• Concurrent acquire verified via thread barrier pattern (2 threads/1 output, 5 threads/5 outputs)
• Basket isolation confirmed (pool outputs excluded from default auto-fund)
• Acceptance criteria verified against HLR [HLR] UTXO Pool Management for High-Frequency Transactions #474

Related: #474