fix!: reject keyword arguments in @model signatures instead of warning

[bvdmitri]

Closes #307. This is a breaking change — see below.

Problem

src/model_macro.jl:959-961 parsed keyword arguments in a @model signature and then ignored them,
emitting 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

A user writing @model function m(x; a = 1) gets no a in the body, so the only real signal is a
later UndefVarError — and the warning is easy to lose in REPL output. Unsupported positional
arguments already hard-error at the call site (model_macro.jl:923), so this was inconsistent.

Change

Error at macro expansion, naming the offending keyword arguments and the supported form:

The `m` model macro does not support keyword arguments in the model signature, but got 2: a, b.
Declare all model interfaces as positional arguments, `m(x)`, they are passed by name at the call site.

Breaking-change assessment

Nothing in this repository relied on the old behaviour:

• the warning string appears in exactly one place and no test or doc asserts it (there is no
  @test_logs / @test_warn anywhere in the suite)
• ms_kwargs is referenced only at model_macro.jl:952 and :959
• zero @model function signatures containing ; exist across src/, test/, docs/ and
  README.md — all 39 @model function occurrences in the docs and README are positional-only

The risk is downstream (RxInfer.jl and user code). Any such definition will now fail to expand — but
its keyword arguments were never in effect in the first place, so the fix surfaces a pre-existing
silent bug rather than removing working functionality. Worth a breaking version bump; there is no
CHANGELOG in this repo to update.

Tests

New test item in test/model_macro_tests.jl asserting the error fires for a kwarg-bearing signature,
that it names the offending keywords and the supported form, and that positional-only signatures still
expand.

Full suite on Julia 1.12: 259/259 test items, all passing.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.76%. Comparing base (4cc7c6c) to head (9a24e99).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
+ Coverage   90.72%   90.76%   +0.04%     
==========================================
  Files          16       16              
  Lines        2263     2264       +1     
==========================================
+ Hits         2053     2055       +2     
+ Misses        210      209       -1     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.