Add challenge family ladders and AVL difficulty suite

[cmccomb]

Summary

• add family/difficulty metadata and filtering for challenge cases
• introduce an easy/medium/hard/insane AVL challenge ladder
• regenerate challenge docs and add coverage for rerun determinism

Validation

• make fmt
• make lint
• PYTHONPATH=src .venv/bin/pytest tests/test_challenges_catalog.py tests/test_challenge_docs.py tests/test_challenges_internals.py tests/test_challenges_runner.py
• PYTHONPATH=src .venv/bin/python scripts/generate_challenge_docs.py --check
• PYTHONPATH=src .venv/bin/python scripts/run_challenges.py --catalog-dir challenges/cases --tier all --family avl --artifact-root artifacts/challenges/avl_family --metrics-json artifacts/challenges/avl_family/challenges_metrics.json --junit-xml artifacts/challenges/avl_family/challenges.junit.xml --summary-md artifacts/challenges/avl_family/summary.md

[cmccomb]

Reopening to retrigger PR checks for the latest head commit after GitHub missed the synchronize event.

[cmccomb]

Superseding with a fresh PR because GitHub stopped attaching new check suites to this PR's updated head commits.