feat(native): bind hoverWithinGroup to a named ancestor group

[Ernxst]

A hoverWithinGroup name always bound to the outermost hoverGroup ancestor, so a descendant nested inside two groups couldn't target the inner one — Tailwind's group-hover/sidebar had no equivalent. Fixes #571.

Add a hoverWithinGroup name that binds hoverWithin to the nearest ancestor whose hoverGroup equals it, instead of the outermost marked ancestor:

<div style={{ hoverGroup: 'outer' }}>
  <div style={{ hoverGroup: 'inner' }}>
    <span
      style={{
        hoverWithinGroup: 'inner',
        hoverWithin: { backgroundColor: '#7c86ff' },
      }}
    />
  </div>
</div>

Hovering outer's own padding leaves the span unstyled; hovering inner styles it. Left unset, hoverWithin keeps following the outermost marked ancestor as before. A hoverWithinGroup naming no ancestor hoverGroup produces a style diagnostic instead of silently doing nothing.

packages/native/src/renderer.rs#L11474-L11487 resolves the binding each frame: unnamed picks the outermost marked ancestor (InheritedHoverGroup::first()); named walks the accumulated ancestor list from the innermost end for the first name match. The same resolution is duplicated for the boolean hover_within used by effective_display, transitions, and the <virtual-list> path, and for the JS-facing TestGpuixRenderer::get_resolved_style used by the React test suite.

Risk is confined to hoverWithin/hoverWithinGroup resolution: unnamed behavior is unchanged (verified by the existing nested-group test), and the new field is additive on StyleDesc.

Harness: Claude Code
Agent: none
Model: anthropic/claude-sonnet-5

Task statements

1. Implement issue hoverGroup names are ignored: a descendant cannot choose which group it follows #571 on Ernxst/gpuix: make a descendant bind to a named ancestor hoverGroup via a new hoverWithinGroup field, matching Tailwind's group-hover/name semantics, while keeping the unnamed hoverGroup behavior unchanged; update the README API section and add a changeset. (Working-directory and workflow instructions omitted.)

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