fix(io): move arrow properties into edge.style for correct G6 rendering

Arrow apply/get functions in EXCEL_EDGE_PROPERTIES wrote to top-level
edge (e.startArrow) but getEdgeStyleOrDefaults reads from edge.style,
so arrows from Excel never reached the rendered graph. Also fixes wrong
DEFAULTS paths in node Label apply, adds type/style defaults to the G6
Graph constructor, and consolidates the double initialize() call.

Adds 54 TDD tests covering all optional node and edge Excel properties
through the full import → style → render pipeline.

Author: Mnikley