refactor(graph-layer): streamline loader data inputs

[ibgreen]

Summary

• export reusable guards for GraphData and columnar graph data so graph layers can detect loader outputs without casts
• refactor GraphLayer data resolution to use the new guards, reuse loader fallbacks, and rely on deduplicated log.warn messaging
• convert the data handling test to static imports with a hoisted window mock that covers both GraphData and ArrowGraphData inputs

Testing

• yarn lint
• yarn vitest --project node --run

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Codex Task