metal : fuse add, mul

[ggerganov]

target #14629

Fuse GGML_OP_ADD and GGML_OP_MUL

LLAMA_SET_ROWS=1 ./scripts/compare-commits.sh master gg/metal-fuse-add -m ./models/qwen3-30b-a3b/ggml-model-q8_0.gguf -m models/gemma-3-4b/ggml-model-q8_0.gguf -fa 1 -t 1

Model	Test	t/s master	t/s gg/metal-fuse-add	Speedup
gemma3 4B Q8_0	pp512	2444.84	2494.63	1.02
gemma3 4B Q8_0	tg128	90.39	96.76	1.07
qwen3moe 30B.A3B Q8_0	pp512	1362.92	1420.74	1.04
qwen3moe 30B.A3B Q8_0	tg128	70.12	76.68	1.09

Testing

make -j && GGML_METAL_FUSION_DEBUG=2 ./bin/test-backend-ops -o RMS_NORM_MUL_ADD -b Metal

Backend 1/3: Metal
  Device description: Apple M4 Max
  Device memory: 28753 MB (28747 MB free)

ggml_metal_encode_node: fuse: RMS_NORM + MUL + ADD
  RMS_NORM_MUL_ADD(type=f32,ne=[64,5,4,3],eps=0.000000): OK
ggml_metal_encode_node: fuse: RMS_NORM + MUL + ADD
  RMS_NORM_MUL_ADD(type=f32,ne=[64,5,4,3],eps=0.000001): OK
ggml_metal_encode_node: fuse: RMS_NORM + MUL + ADD
  RMS_NORM_MUL_ADD(type=f32,ne=[64,5,4,3],eps=0.000100): OK
ggml_metal_encode_node: fuse: RMS_NORM + MUL + ADD
  RMS_NORM_MUL_ADD(type=f32,ne=[64,5,4,3],eps=0.100000): OK
ggml_metal_encode_node: fuse: RMS_NORM + MUL + ADD
  RMS_NORM_MUL_ADD(type=f32,ne=[64,5,4,3],eps=1.000000): OK
  6543/6543 tests passed
  Backend Metal: OK
ggml_backend_metal_device_rel: fused ADD: 5
ggml_backend_metal_device_rel: fused MUL: 5

---

• Disable with env variable
• Print fuse stats
• Fuse with norms, cpys, etc.
• Cleaner kernel impl?