bug fixes for running on metal

[yogthos]

Overview

• kernels/quantize.h was missing quantize_q2_0s and quantize_q2_1. The kernel instantiations referencing them were added, but the functions themselves were never written, so the quantize Metal library failed to compile at runtime. Ported the implementations from ggml-quants.c (quantize_row_q2_0s_ref / quantize_row_q2_1_ref).
• dequantize_iq4_nl assigned floats directly into a bfloat matrix element, which the Xcode 26.1 Metal compiler rejects. Now computes into a float4x4 temp and casts once at the end.
• kernel_get_rows_f copied between mismatched scalar types (float ↔ bfloat ↔ half) without a cast. Added (T) on the assignment.

Requirements

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• AI usage disclosure: YES