Compact README ecosystem-positioning table for faster scanning

[Copilot]

The README related-work section was too wide and verbose, making comparisons hard to scan quickly. This change compresses the table while preserving the core positioning signal for each comparator class.

• What changed

  • Reduced the ecosystem table from 5 columns → 3 columns:
    • Comparator
    • Focus
    • py3plex 2.0 relation
  • Rewrote row content to short, high-signal summaries.
  • Consolidated adjacent tools into grouped rows where distinctions were repetitive (e.g., temporal engines, general graph stacks, graph-ML frameworks).

• Resulting structure

  • Lower visual width in GitHub markdown.
  • Faster read path: tool family → scope → py3plex differentiation in one pass.
  • Maintains explicit py3plex 2.0 row as the anchor.

| Comparator | Focus | py3plex 2.0 relation |
|---|---|---|
| **NetworkX / igraph / graph-tool** | General-purpose graph analysis stacks | Core graph ecosystem overlap; py3plex adds layer-aware abstractions and reproducible multilayer workflows. |
| **PyTorch Geometric / DGL** | Graph deep learning frameworks | Adjacent graph-ML layer; py3plex focuses on preparation, querying, and scientific analysis workflows. |