v0.2.1 — full-format hardening

19 correctness/security fixes across docx/xlsx/pptx from a full three-way review: docx preview XSS sanitization, edit-result ordering, negative-index guards, hyperlink-safe replaces, markdown table fidelity, xlsx spec validation, honored fail_on_quality_error, template×format validation, 404s for missing bytes, Redis SSE tail repair, per-format asset kinds.