The Metal backend lacked GGML_ROPE_TYPE_MROPE/IMROPE support, so models
whose GGUF carries rope_sections (e.g. Qwen 3.5 hybrids) could not run
fully offloaded on Apple Silicon.
Add rope_multi_f32/f16 kernels with section-based position handling
(t/h/w/e blocks, 4 position ids per token), imrope's interleaved section
selection, and the corresponding dispatch in ggml-metal.m. Vision-mode
mrope is not implemented and is asserted out explicitly.
Validated on M2: kernel output matches the CPU backend, and Qwen 3.5-9B
(Q4_K_M, -ngl 99) WikiText-2 perplexity over 145 chunks lands within
0.006 of the same model's CPU baseline.
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