[BUG] `useTransform` value desyncs when source is set inside an effect under React 18 Strict Mode

Summary

Updating a MotionValue inside a (layout) effect during Strict Mode's mount→unmount→remount cycle leaves a useTransform-derived value permanently stuck at its stale value.

Root cause

useCombineMotionValues recomputes via two paths: a sync updateValue() in the render body, and an async frame.preRender(updateValue) on the source's change, cancelled by cancelFrame in the effect cleanup.

https://github.com/motiondivision/motion/blob/61833240e899fbbe4f50484ec5f9f7fe688de843/packages/framer-motion/src/value/use-combine-values.ts#L22-L42

Failing sequence under Strict Mode:

1. First effect sets source → change → updateValue scheduled to next frame (still stale).
2. Strict Mode cleanup runs cancelFrame → pending task cancelled before it flushes (remount is continuous, no frame in between).
3. Remount re-subscribes but doesn't re-render, so the sync fallback never runs again.
4. Effect sets the same value → set short-circuits → no change → nothing re-scheduled.

Result: derived value stays at f(oldValue) while source is newValue.

React 19 behavior changes

This bug disappears in React 19:

In React 18, Strict Mode's mount→unmount→remount runs synchronously inside the commit phase, so the whole effect → cleanup → effect sequence finishes before any frame. Derived value's update task is canceled and never runs.

In React 19, the double-invoke was moved into the passive-effects phase. The order becomes effect → rAF → cleanup → effect. The derived value update task can run before cleanup so the bug disappears.

Reproduction

https://codesandbox.io/p/sandbox/snowy-river-qq5y9c

Expected vs actual

Expected: doubled equals 20. Actual: doubled equals 0, never recovers.

Start with packages/framer-motion/src/value/use-combine-values.ts at the linked lines 22-42, then reproduce the issue in the linked CodeSandbox under React 18 Strict Mode. Trace the effect, cleanup, frame scheduling, and remount sequence; done means the reproduction's derived value reaches 20 instead of remaining at 0 and stays synchronized.