feat: ASCII/Unicode graph renderer for terminal visualization

Adds GraphAsciiRenderer with three render modes:
- Spatial force-directed layout on character grid (ASCII or Unicode)
- Adjacency list view with tree-style formatting
- Degree histogram as horizontal bar chart

Features: configurable grid size, node highlighting by name or degree,
degree annotations, edge type legend, file export.

Usage: new GraphAsciiRenderer(graph).render()

Author: sauravbhattacharya001