Create a P&ID arrow

A first-class arrow entity in a schematic. Bbox-only entity that lives in pid.arrows. Arrows are flow-direction hints for the inference algorithm; they participate in flow.py's geometry-driven assignment, not in schematic.nodes topology. heading is the way the head points, read from its ink at detection time; None when the ink did not read as a head, in which case flow inference falls back to the geometry around the box.