fix: support linux aarch64 dependencies

[TATP-233]

Summary

• add Linux aarch64 as a uv required environment so mujoco-uni aarch64 wheels are locked
• route Linux aarch64 Torch to the cu130 r2 index while keeping Linux x86_64 and Windows on cu128
• update the Torch source contract test for the split platform markers

Validation

• uv lock --check
• uv run pytest tests/scripts/test_torch_cuda_source.py
• make test-all
• On NVIDIA DGX Spark / Linux aarch64 host: uv run --frozen --extra mujoco python -c "import platform; import mujoco; print(platform.system(), platform.machine()); print(mujoco.__version__)"
• On NVIDIA DGX Spark / Linux aarch64 host: uv run --frozen python -c "import platform; import torch; print(platform.system(), platform.machine()); print(torch.__version__)"