Fix gated delta kernel precision

[kernelpool]

Adds Kahan compensation and unrolls loops. Fixes #1061

mlx-community/Qwen3.5-35B-A3B-4bit

Context	Before pp	After pp	Δ pp	Before tg	After tg	Δ tg	Before Mem	After Mem	Δ Mem
0.5k	2040.1	2039.2	-0.0%	82.1	83.6	+1.8%	20.24 GB	20.24 GB	0.0%
1k	2574.4	2551.9	-0.9%	81.7	84.1	+2.9%	20.57 GB	20.57 GB	0.0%
2k	2830.8	2818.4	-0.4%	82.2	85.8	+4.4%	21.40 GB	21.40 GB	0.0%
4k	2905.8	2889.3	-0.6%	85.0	84.8	-0.3%	21.66 GB	21.65 GB	0.0%
8k	2845.5	2827.4	-0.6%	84.2	84.3	+0.2%	22.02 GB	22.02 GB	0.0%
16k	2627.5	2614.6	-0.5%	83.3	84.1	+1.0%	22.70 GB	22.70 GB	0.0%
32k	2245.7	2235.4	-0.5%	80.3	79.9	-0.4%	23.96 GB	23.96 GB	0.0%
Avg	2581.4	2568.0	-0.5%	82.7	83.8	+1.4%
PPL	4.14	4.14	0.0%