[ZIP Draft] ZSV: one ZEC-backed stablecoin on Zcash

The unnumbered Draft is submitted in zcash/zips#1438 ; this post opens the wider community discussion. Why native borrowing? ZEC holders may want dollar-denominated liquidity while retaining ZEC as collateral. ZSV proposes borrowing and payment rules enforced on Zcash, with one protocol-defined asset and no arbitrary token-launch interface. Specifics (v1) Locks ZEC in a dedicated vault value pool. Issues zUSD when an owner borrows against sufficient collateral. Burns existing zUSD on repayment or liquidation, accounting for any uncovered debt. Allows public zUSD payments and permissionless liquidation. Accepts ZEC/USD reports signed by five of seven upgrade-selected reporters, with an observation anchored to a recent Zcash block. Vaults and payments are public first, and a follow-up ZIP would enable shielded vaults. There is no arbitrary asset registration, issuer key, governance token or general execution environment. This restriction cannot prevent off-protocol derivatives or speculation. Every operation in a block u