PORTLAND, Maine (AP) — A battle over a $1 billion transmission line that won all regulatory approvals only to be rebuked by state residents in a referendum now comes down to nine regular folks. In a rare move, a jury is being asked to decide a complicated constitutional matter — whether developers have a vested right to complete the 145-mile (233-kilometer) project, which would supply Canadian hydropower to the New England power grid. The constitutionality of the statewide referendum on the project depends on the jury’s decision on the narrow vested-rights issue.