Expand README: add student benefits and why use this over ChatGPT

[om952]

The current README covers the basic workflow and architecture. We should expand it to address:

1. Student benefits — what specific advantages does this DSA engine offer to students learning algorithms?
2. Comparison with ChatGPT — why use this tool instead of asking ChatGPT directly? (e.g. structured output, step-by-step visualizer, deterministic verification, no API key management for students)
3. Key differentiators — visualizer, deterministic engine for sorting/BFS, retry-repair loop, no hallucinated code

This helps new users (especially students) understand the value proposition immediately.