Criticality bucket for SchedContext. Phase 110.C uses this to
pick which BucketedFifoSet / BucketedEdfSet slot a callback
dispatches through; later phases (110.D) map it to OS priority.
Default Normal keeps existing single-bucket workloads unchanged
— every default-Fifo SC sits in Normal, so dispatch order is
bit-identical to pre-110.C when no callback opts in to Critical
or BestEffort.
Single-thread non-preemption note: a BestEffort callback already
running blocks Critical work that becomes ready mid-cycle. Hard-
RT scenarios need 110.D’s multi-executor preemption.
Highest-priority bucket. Drained first within a single
spin_once cycle; non-preemptive against in-flight lower-
priority callbacks (see Phase 110.D for preemption).