feat: add callflow HTML export

[PorunC]

Summary

• add graphify export callflow-html for generating a self-contained Mermaid architecture/call-flow HTML page from graphify-out/graph.json
• group communities into architecture-oriented sections and render overview diagrams, section flowcharts, key files, call detail tables, hyperedges, stats, and graph report highlights
• wire the exporter into the CLI, document the command in README/ARCHITECTURE, and add focused tests for generation, CLI usage, section grouping, and HTML escaping

Screenshot

Note: the screenshot is referenced in this PR description only and is not committed to the repository.

Tests

• pytest -q tests/test_callflow_html.py