The contact sheet, the dedup comparator and the drill-down each validated JPEG data-URIs independently. They now share src/lib/guardrails/videoBridgeFrameContract.ts. No behaviour change; the sibling tests that asserted a per-module message were aligned to the shared one. Closes the Standards-4 residue from the 2026-08-18 Video Bridge review.
Verified in a combined worktree with three sibling PRs of this batch: typecheck:core clean, 134/134 focused tests (4 skipped), i18n UI coverage PASS across all 42 locales.