Add GEXF import support

[lmeyerov]

Summary

• add GEXF loader with viz attribute bindings (color/size/position/opacity/thickness) and validation for missing nodes/edges
• add GEXF export (to_gexf) with viz/attribute mapping
• map node viz shapes to FA4 icons (disc→circle, square→square, triangle→caret-up, diamond→diamond, image→uri or picture-o)
• add fixtures + tests for 1.1draft/1.2draft/1.3 GEXF variants, failure cases, and export roundtrip
• add docs + demo notebook

Testing

• uv run pytest graphistry/tests/test_gexf.py -x

Plot URL

• https://hub.graphistry.com/graph/graph.html?dataset=80d5bafb53514a0586a1dd3e31da0688&type=arrow&viztoken=bf4b36f5-0e49-4cc2-92cd-ea59f2f127ae&usertag=ef9e6f8d-pygraphistry-0.50.5+0.g6a272f1f.dirty&splashAfter=1769404440&info=true&play=0

Fixes #899

[lmeyerov]

need attribution?