* llama : do not map foreign ranges into a backend buffer

A buffer created from a host pointer pins everything that it wraps. When
several contexts share the same file (lazy tensors, MoE tensors kept on the
CPU with -cmoe), the single buffer that spans the first and the last tensor
of a context also wraps the ranges owned by the other contexts, pinning data
that the backend never reads.

For models with per-layer token embeddings this pins the whole lazy tensor
into the GPU buffer (27 GiB for Qwen3.8-Flash-Next IQ4_NL), defeating the
point of reading it lazily.

Instead, compute the file ranges of each context and subtract the ranges
owned by the other contexts, then create one buffer per range. Weights that
are mapped in more than one context (e.g. token_embd.weight) keep being part
of the range of every context that uses them.

Assisted-by: pi:llama.cpp/Qwen3.8-Flash-Next

* llama : enable prefetch on macOS

The prefetch hint of the tensors marked as prefetchable is a no-op on macOS,
so their pages are faulted in one by one while the graph runs. madvise
(MADV_WILLNEED) behaves as expected here, so add the platform to the guard.

With the lazy per-layer token embeddings of Qwen3.8-Flash-Next IQ4_NL read
from a cold file cache, prompt processing goes from 356 / 375 / 475 t/s to
735 / 749 / 788 t/s for pp512 / pp2048 / pp4096.

Assisted-by: pi:llama.cpp/Qwen3.8-Flash-Next

* llama : use a plain buffer list per context

Assisted-by: pi:llama.cpp/Qwen3.8-Flash-Next