Adds a CWE-79 regression test: a sidecar entry whose displayName contains
an <img onerror=alert(1)> payload must round-trip through deserialize and
the Tiptap renderHTML without producing a real <img> element in the editor
DOM. Locks down the "renderHTML's third tuple entry is a text node, never
parsed as HTML" invariant so a future "use innerHTML for performance"
refactor cannot silently regress.
Nora #5618 detection-gap concern.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>