proactiveDustSync

Keep dust pre-synced in the background (#235). When enabled, the SDK runs a live tip-advance dust subscription so a transaction never waits on a cold sync; progress is observable via MidnightWallet.syncStatus (and surfaced by the foreground-service Live-Update notification). Off by default — opt in for wallet-first apps. Delta only; the first cold sync is still O(chain) but now runs ahead of need.