M25 — VS-NfD: hardening & supply chain

The items pulled forward from the roadmap plus supply-chain evidence: attachment integrity hashes, a hard plugin off-switch, a stable SIEM event catalogue, SBOM in CI, image digest pinning, and a pinned Node version. Digest pinning (#203) should land before M28 (offline/airgap deployment) — the mirror procedure and the offline update path both build on immutable references.

No due date
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