Dequeue by Shifting Left

ShiftQueue keeps the front of the queue at index 0, so removing it means sliding every later value down one slot. Only dequeue() is missing, and its empty guard plus the line that saves the front value are already written.

Requirements:
1. Move items[1] through items[size - 1] one slot to the left.
2. Decrease size and return the saved value.

Do NOT write a main method.