Constant NO_VECTOR_OWNER 

const NO_VECTOR_OWNER: usize = usize::MAX; // 18_446_744_073_709_551_615usize

Per-hart ownership of the live Vector register file.

When a task that used the V extension is rescheduled on the same hart, we can skip restoring vregs if it still owns the live state. This removes a very expensive per-timeslice illegal-instruction trap for vector-heavy workloads.