Setting innerHTML="" on a mock element now detaches and removes all
children, matching browser behavior. Previously hs-cleanup! (which
sets body.innerHTML="") left stale children attached, causing
querySelector to find elements from prior tests.
Also clears children when textContent is set (browser behavior).
375 → 393/831 HS tests (+18).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>