Session delivery ordering

Chose explicit steer/move delivery ordering over generic FIFO because safe-step and idle boundaries have different priorities. Prompts steer by default; at safe-step boundaries, steered compaction runs through the first steered move, while other steers retain enqueue order. At idle, steers take priority; otherwise deliver exactly one queued item before reevaluating continuation. User promotion resets step allowance; a steer batch resets it once.