Consider promoting `@model` keyword-argument drop from `@warn` to `@error`

[docxology]

Summary

src/model_macro.jl:959-961: keyword arguments in a @model function signature are parsed
and then ignored, with only a warning:

if !isnothing(ms_kwargs) && length(ms_kwargs) > 0
    @warn("Model specification language does not support keyword arguments. Ignoring $(length(ms_kwargs)) keyword arguments.")
end

Suggestion

A user writing @model function m(x; a=1) gets no a in the body; a later UndefVarError
is the only signal, and the @warn is easy to miss. Since unsupported positional
arguments already hard-error (model_macro.jl:923), consider promoting the kwarg case to an
error for consistency and earlier failure. Deliberate design choice — flagging for
discussion rather than a hard bug.