Add Metal (Apple Silicon) build variants

Use shared kernels-test-utils and set metal3.1 compatibility

Add Metal CI and point kernel-builder to ChipFlow fork

Improve fp32 precision in rotary embedding Metal kernel

Fix MPS command encoder lifecycle for sequential kernel calls

Fix test reference: use scalars to avoid PyTorch view aliasing bug

Migrate to embedded metallib and update flake.lock

Fix test: capture CPU reference tensors before comparison

Add Metal rotary embedding kernel matching vLLM interface