feat: validate independent downstream consumers

[codeforester]

Summary

• publish an adopter readiness checklist, migration path, support channel, and friction/release disposition register
• add three independent downstream compatibility fixtures: Atlas (Click), Beacon (Typer), and Cinder (automation/JSON)
• retain per-consumer packaging, documentation, tests, and a wheel-first compatibility workflow
• validate the consumer manifest and include downstream evidence in source distributions and repository documentation

Validation

• python3 scripts/validate_consumers.py
• python3 scripts/validate_examples.py
• python3 scripts/validate_docs.py
• .venv/bin/python -m pytest -q
• all three downstream consumer suites via PYTHONPATH=lib/python:compatibility/consumers/<consumer>/src
• ruff check and ruff format --check
• ./tests/validate.sh

Closes #72