Checkbox crashed on RN: its checkmark was a raw <svg><polyline> (no native component) and it couldn't toggle (RSD only makes <button> a Pressable — onClick on label/div/span never fires — and has no <input type=checkbox>). Native branch now: an html.button (the pressable row, role=checkbox + aria-checked) containing html.div box + a CSS-border checkmark (rotated right/bottom borders — no react-native-svg, so zero web bundle cost), synthesizing the onChange payload. Box/checkmark use html.div (View) not html.span (Text) so they lay out; nativeButton strips RSD's default <button> border/bg/padding. Switch: the unimplemented <input type=checkbox> is gated to web and a11y (role=switch/aria-checked) moves to its button on native.

Web is unchanged by construction: the web branch is the original label+input+span+svg verbatim; everything new is isNative-gated. Verified on an iOS sim — both render, toggle on tap, and fire onChange/onCheckedChange; no crashes or 'not implemented' errors.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>