feat(cat5): representative cycles alongside Betti-1

[jphein]

Summary

Closes #16.

• Adds representative_cycles: list[list[str]] to GapDetectionReport — up to 10 shortest cycles from nx.cycle_basis() on the largest component's undirected projection
• Sorted by length then alphabetically for deterministic output
• format_report() renders cycles when present (e.g., A → B → C → A)
• Only computed when run_homology=True and the largest component has edges; zero cost when skipped

Test plan

• test_representative_cycles — verifies cycle count and node membership against fixture
• test_format_report_shows_cycles — verifies rendered output contains cycle arrow notation
• Existing Cat 5 tests pass unchanged

🫏 Generated with Claude Code

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[M0nkeyFl0wer]

Ship. Representative cycles from nx.cycle_basis() on the largest component is the right scoping — Betti-1 alone tells you "there are N holes" but not "where to look," and the top-10 shortest cycles is the actionable form. Deterministic sort handles the reproducibility piece. Clean.