fix: do not scan custom wrappers for GleamView.Line

[RamboWasReal]

Summary

Regression fix for #7 (cursor/improvements-audit-6a43).

scanLineChildren walked every component's props.children. That is too broad: a parent GleamView entered Line mode (plain View, no block shimmer) when GleamView.Line appeared in a custom wrapper's children even if those Lines:

• bind to a nested GleamView (nearest-ancestor contract), or
• are never mounted (hidden tab/panel, etc.).

On main, only direct children and Fragments were inspected; custom wrappers used registerLine after mount. First-render detection is kept for the documented case (<View> / Fragment). Custom components are left to registration.

Repro (before this fix, on #7)

function SkeletonCard({ children }) {
  return <GleamView testID="inner" loading>{children}</GleamView>;
}

<GleamView testID="outer" loading>
  <SkeletonCard>
    <GleamView.Line><Text>Title</Text></GleamView.Line>
  </SkeletonCard>
</GleamView>

Expected (main + docs): outer stays a native shimmer; Line binds to inner.
Actual on #7: outer became a plain View (loading undefined) — parent shimmer gone.

What changed

• Recurse only through React.Fragment and View
• Tests for nested-GleamView composition, unmounted Line children, View vs custom wrappers
• README / CHANGELOG wording aligned with that scope

No native iOS/Android changes. No new JS dependencies.

Verification

• yarn test — 100 passed
• yarn typecheck
• yarn lint
• yarn prepare

Residual risks on #7 (not fixed here — not regressions vs main)

• useReduceMotion() starts false, then flips async → NativeGleamView remounts to static View (ref churn; onTransitionEnd not on the static path)
• Line-mode parent accessibilityState is applied then overwritten by {...viewProps} (busy is still set on Lines)
• gleamAccessibilityState forces busy: true while loading, including over a caller busy: false