Expand R ground truth to k>1 multi-kinship and random.slope

[vineetver]

Code is bit-identical k=1 parity tested in src/staar/multi_kinship.rs:1001 against R GMMAT::glmmkin.multi.ai. Need fixtures for k>=2 multi-kinship and --random-slope to lock parity at the actual production shape.

What's missing

• src/staar/testdata/ground_truth.json has ai_staar and multi_staar sections, no multi_kinship or random_slope cases.
• The R generator scripts/r/generate_staar_ground_truth.R was deleted in 08a9df2 when the single-trait fixtures were locked. Restoring it (or a successor) is the simplest path.

Done when

• Ground truth JSON includes at least one k=2 multi-kinship case and one random.slope case.
• cargo test --all-targets runs the new parity tests and they pass.
• The generator script is checked back in under scripts/r/ so fixtures are reproducible.

Background

• v0.3.0 shipped multi-binary, multi-kinship, and random.slope nulls in staar: multi-binary, multi-kinship, and random.slope nulls #133.
• This is coverage validation, not code change. Code already exists.