Unify graph runtime adapters behind GraphInterface

[ibgreen]

Summary

• replace runtime detection with new GraphInterface adapters, including TabularGraph and LegacyGraph implementations plus a LegacyGraphLayout bridge
• refactor GraphEngine to operate solely on GraphInterface/GraphRuntimeLayout and expose a stylesheet factory for consumers
• update the edge attachment helper to request stylesheet engines through the refactored GraphEngine API

Testing

• npx eslint modules/graph-layers/src/core/runtime-types.ts modules/graph-layers/src/core/graph-engine.ts modules/graph-layers/src/layers/edge-attachment-helper.ts
• git commit -m "Refactor graph runtime to interface adapters"

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Codex Task