fix: enforce OFF full-text provider contract

[maxjustships]

Summary

• route OFF free-text resolution exclusively through the documented v1 CGI contract; v2 is retained only for product/barcode reachability
• surface separate OFF product/barcode and full-text readiness in setup and nomnom doctor --json
• add replayable endpoint, parameter, semantic-query, retry/Retry-After, no-fallback, and confidence guardrail coverage

Validation

• PYTHONPATH=. pytest -q (126 passed)
• ruff check .

Closes #17