Simplify scroll restoration with shared ScrollRef on CacheNode

[acdlite]

Replaces the segment-path-matching scroll system with a simpler model based on a shared mutable ScrollRef on CacheNode.

The old system accumulated segment paths during navigation and matched them in layout-router to decide which segments should scroll. This was necessary when CacheNodes were created lazily during render. Now that we construct the entire CacheNode tree immediately upon navigation, we can assign a shared ScrollRef directly to each new leaf node. When any segment scrolls, it flips the ref to false, preventing other segments from also scrolling. This removes all the segment path accumulation and matching logic.

Fixes a regression where calling refresh() from a server action scrolled the page to the top. The old system had a semantic gap between null (no segments) and [] (scroll everything) — a server action refresh with no new segments fell through to a path that scrolled unconditionally. The new model avoids this: refresh creates no new CacheNodes, so no ScrollRef is assigned, and nothing scrolls.

Repro: https://github.com/stipsan/nextjs-refresh-regression-repro

There is extensive existing test coverage for scroll restoration behavior. This adds one additional test for the server action refresh bug.

[nextjs-bot]

Tests Passed

[acdlite]

Note: the diff for layout-router.tsx looks larger than it actually is — git's diff algorithm doesn't align the two scroll handlers well because the guard condition at the top changed shape. Here's what actually changed:

In both InnerScrollAndFocusHandlerOld and InnerScrollHandlerNew:

The old scroll-decision logic:

const { focusAndScrollRef, segmentPath } = this.props

if (focusAndScrollRef.apply) {
  if (
    focusAndScrollRef.segmentPaths.length !== 0 &&
    !focusAndScrollRef.segmentPaths.some((scrollRefSegmentPath) =>
      segmentPath.every((segment, index) =>
        matchSegment(segment, scrollRefSegmentPath[index])
      )
    )
  ) {
    return
  }
  // ... scroll execution (unchanged) ...
  focusAndScrollRef.apply = false
  focusAndScrollRef.hashFragment = null
  focusAndScrollRef.segmentPaths = []
}

Was replaced with:

const { focusAndScrollRef, cacheNode } = this.props

const scrollRef = focusAndScrollRef.scrollRef ?? cacheNode.scrollRef
if (scrollRef === null || !scrollRef.current) return
// ... scroll execution (unchanged) ...
scrollRef.current = false
focusAndScrollRef.hashFragment = null

Other changes:

• segmentPath prop removed from ScrollAndMaybeFocusHandler (replaced by cacheNode)
• matchSegment import removed (no longer needed)
• OuterLayoutRouter passes cacheNode instead of segmentPath to the scroll handler

Everything else in the file (the scroll execution logic inside disableSmoothScrollDuringRouteTransition, focus handling, DOM node finding) is unchanged.