Fixes the 44 failing subtests of WPT /dom/ranges/Range-collapse.html
(142/186 -> 186/186): per the DOM IDL, collapse(optional boolean
toStart = false) collapses the range to its END point when the argument
is omitted; we defaulted to the start.
Internal callers (deleteContents) already pass an explicit true and are
unaffected.
Coverage: /dom/ranges/Range-collapse.html 142/186 -> 186/186 (fully
green).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>