Several motion-dom functions accessed `window` without a `typeof window`
guard, throwing in non-browser JS runtimes — or when `window` is shadowed
to undefined by an IIFE wrapper (e.g. Lynx's web runtime). The immediate
crash reported was `animateTarget` reading `window.MotionHandoffAnimation`,
but the same unguarded access existed across projection, keyframe
resolution, computed-style reads, gestures, resize handling and the
scroll-timeline support checks.

Add `typeof window !== "undefined"` guards at each site, matching the
existing pattern in render/utils/reduced-motion. `typeof` never throws on
an undefined binding, and every guard evaluates to true in a browser, so
existing behaviour is unchanged.

Fixes #3735

Co-Authored-By: Claude Opus 4.8 <[email protected]>

Limit non-browser handling to animateTarget and use a module-level isBrowser flag so the animation loop avoids repeated environment checks.

Exiting children were spliced into the new children array at their index
within the previously rendered children. Those are two different index
spaces, so exiting children could be interleaved with entering children
and push present children to new positions, remounting them (#3746).

Reinsert each exiting child directly after the child it previously
followed instead.

Co-Authored-By: Claude Opus 5 <[email protected]>

Guard animateTarget window access in non-browser runtimes (#3735)

…-strictmode-remount

Fix AnimatePresence remounting present children inside React.StrictMode (#3746)