Add GraphML loader for graph-layers

[ibgreen]

Summary

• add a GraphML loader that maps nodes, edges, keyed data, and defaults into TabularGraph instances
• cover the loader with fixtures and Vitest specs that verify attribute typing and directed edge handling
• document supported GraphML constructs in the graph-layers README and add the fast-xml-parser dependency

Testing

• yarn lint
• yarn vitest run modules/graph-layers/test/loaders/graphml-loader.spec.ts (fails to launch Playwright browsers in this environment, tests still pass)

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Codex Task