fix: make PubChem CAS resolution conservative

[rastagan-git]

Summary

• Add repeatable --skip-pattern REGEX handling so structural workbook rows can be excluded before any PubChem request.
• Separate provider execution from CAS curation with PubChem CAS Resolution, PubChem CAS Candidate Count, and configurable Resolved CAS output.
• Populate a resolved CAS only for a confirmed CAS query or exactly one checksum-valid candidate; preserve multiple or missing candidates without choosing the first value.
• Document the conservative workflow in both READMEs and the $curate-aroma-data skill.
• Bump the package version to 0.3.1.

Root cause

The workbook workflow sent every identifier-shaped cell to PubChem and exported provider status together with raw CAS candidates. A valid PubChem record could therefore look like a resolved identity even when the row was structural, or when the record contained multiple or zero defensible CAS candidates.

Validation

• ruff check .
• ruff format --check .
• pytest -q — 56 passed
• Official skill validator — passed
• Independent skill forward test — passed without network requests or file writes
• Wheel build — aromanexus-0.3.1-py3-none-any.whl; canonical and legacy namespaces plus both console entry points verified

Closes #6