Rewires briefwechsel-rows.visual.spec.ts against the shared fixture
(seedBilateralPair + cleanupBilateralPair), adds afterAll cleanup,
and folds the conv-person-bar visibility gate into openBilateral()
so both the structural test and the snapshot block fail loudly on
a hero-state regression — matching the a11y spec's safety net.
Refs #305
Fixes @saraholt follow-ups 1 + 2 + 3 from PR round-2 review
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>