docs: establish permissioned adoption evidence process

[codeforester]

Closes #196

Summary

• document the permissioned adopter program and distinguish customer evidence from maintained fixtures
• record the honest current status: zero confirmed independent adopters, target three
• emit dated, revision-bound compatibility evidence artifacts from the reference-consumer workflow
• correct compatibility wording so Base and adjacent repositories are not counted as external adoption

Validation

• python scripts/record_compatibility_evidence.py
• python scripts/validate_consumers.py
• python scripts/validate_docs.py
• ./tests/validate.sh
• python -m mkdocs build --strict
• ruff check and format