Previously, `lib/top-level.nix` used an `_isExtended` flag to indicate
whether the provided `lib` was already extended. This made bootstrapping
unclear and introduced the possibility of circular construction.
All construction now flows through the lib-overlay, removing
`_isExtended`. As a result, `lib/top-level.nix` always receives the
final extended lib.
The `<flake>.lib.nixvim` output is now defined as:
(lib.extend <nixvim>.lib.overlay).nixvim
The overlay now imports `top-level.nix` directly, making it the
canonical entrypoint for constructing Nixvim's section of the lib.
A clarifying doccomment was added to `lib/top-level.nix`.
Rename the overlay arguments from the conventional `final: prev:` to the
more explicit `lib: prevLib:`, reflecting that this overlay operates
only on the Nixpkgs lib, rather than a full package set.
Add a doccomment explaining how the overlay is intended to be applied
(via `lib.extend`) and what each argument represents.
Improved inline comments in the overlay.
