Prefer visible folder children when deriving navigation indexes

[Copilot]

Hidden docs referenced inside folders without an index.md could become the folder’s effective index, which caused them to surface in root navigation as the folder link/title. This change makes folder/index resolution prefer visible children and keeps folders hidden when all of their children are hidden.

• Navigation index selection

  • Update QueryIndex to choose the first visible child before falling back to a hidden one.
  • Preserve existing behavior for fully hidden groups by still allowing a hidden fallback when no visible child exists.

• Folder / TOC hidden-state propagation

  • Make folder and nested TOC nodes inherit Hidden from their resolved index.
  • This prevents a folder with only hidden children from appearing as a visible navigation node.

• Regression coverage

  • Add focused tests for:
    • a folder without index.md whose first child is hidden and second child is visible
    • a folder without index.md whose only child is hidden

- folder: user-guide
  children:
    - hidden: autopilot.md
    - file: getting-started.md

With this change, the folder resolves to getting-started.md for its title/URL instead of surfacing autopilot.md.