Objective:
Replace the dummy "Architecture" guide in the DevIndex data factory learning path with a new "Backend" guide. This guide explains the "Fat Client" twist of the DevIndex application—specifically that it lacks a traditional backend server and instead relies on streaming a static users.jsonl file.

Tasks:

• Create learn/guides/devindex/Backend.md.
• Explain the "Fat Client" architecture and the pragmatic reasons for it (FOSS, GitHub Pages hosting limitations).
• Describe the hyper-optimized NDJSON data format.
• Explain the streaming architecture using Neo.data.proxy.Stream, ReadableStream, and progressiveChunkSize.
• Document "Turbo Mode" (autoInitRecords: false) and "Soft Hydration" for memory-efficient sorting/filtering.
• Detail the zero-overhead virtual fields feature using Neo.data.RecordFactory.
• Update learn/guides/devindex/tree.json to link the new guide.