Coin selection: prefer single notes for fastest ZK proofs

Subtle wallet UX win: when a user has both a 5 BTC note and three 1.66 BTC notes, the wallet would pick whichever combo summed closest to the target output. That’s optimal for change minimisation — but proof generation time scales linearly with input notes, and a 1-input proof is 3-5× faster than a 3-input one on the same hardware.

New rule: prefer the smallest set of notes that sum to ≥ target, breaking ties by largest single note. The user feels the difference immediately — proof goes from 4s to 1s on a M1 MacBook.