NEON backend for VectorF32 on AArch64: one float32x4_t per vector
(Len * Packing <= 4), with a vectorised sin/cos port of the SSE helpers.
VectorF16 gets a native float16x8_t path when the target has ARMv8.2-FP16
arithmetic. Without it VectorF16 aliases VectorF32, which converts
fp16 <-> fp32 on Load/Store.
The scalar fallback gains the batched Dot that MatrixRowMajor relies on.
Test tiers added for armv8-a and armv8.2-a+fp16.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
