When innerHTML is set on a mock element, textContent now updates to
match (with HTML tags stripped). Many HS tests do `put "foo" into me`
(which sets innerHTML) then check textContent. Previously textContent
stayed empty because only innerHTML was updated.
Also fixes innerHTML="" to fully detach children from parent.
393 → 408/831 HS tests (+15).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>