[T-054] Office image variant (Trivy-gated) + generated adversarial-input suite

[Ztein]

The Office conversion path ships (LibreOffice headless, sandboxed at process level); remaining: the separate opt-in image variant with LibreOffice (Trivy hard gate green) and the deterministically generated hostile-input tests (macro/OLE/DDE/zip-bomb/truncated).

Canonical ticket: T-054