Go driver v0.1.1: authoritative control snapshots

Adds Client.Snapshot for recovering the current public receipt and existing cursor from the trusted local ledger after status or cancellation. The operation validates receipt authority, uses the existing run lease, and observes the caller deadline with a 15-second ceiling. It performs no BigQuery request and does not renew execution approval or budgets.

Independent review found no issues. Race tests, vet and hosted Linux, macOS and Windows checks passed. CLI integration, JavaScript parity and live BigQuery acceptance remain separate gates.