The lib-overlay was previously wrapped using `makeOverridable`, but its
only input is the flake itself, which is not intended to be overridden.
Remove the wrapper and expose the overlay as a plain function.
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 `lib/default.nix` to `lib/top-level.nix` to make its purpose
explicit and discourage importing it directly. Consumers should obtain
the extended lib via flake outputs or the overlay, rather than bypassing
the required bootstrapping logic.
This clarifies the file's role and avoids suggesting it is the primary
entrypoint to Nixvim's lib subsection.
