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.
llama-dflash.cpp calls ggml_backend_is_metal() and
ggml_backend_metal_set_n_cb() inside an #ifdef GGML_USE_METAL block but
never includes ggml-metal.h, so any Metal-enabled build fails to
compile. Add the same guarded include llama.cpp already uses.
