Coverage: Firefox AMO fetcher, deeper MV3, CVE correlation

[richardmhope]

Proposal

• Firefox AMO fetcher (public REST API) via the get_fetcher factory.
• Deeper MV3 analysis in the inspector.
• CVE/advisory correlation.

Acceptance

• A Firefox AMO extension fetches + scores end-to-end.

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From the PLAN.md roadmap to a SOC-consumable Marvin.