llama: llama_prefetch_rows (#29599)

* llama: llama_prefetch_rows

* llama: support row prefetch on Windows

Apply the Windows port contributed by @praneshgo unchanged.

Source: https://github.com/ggml-org/llama.cpp/pull/29599#issuecomment-5887721014

* avoid exposing llama-mmap in model code, route via llama-impl

* add windows check, only prefetch in lazy mode

* cont : clean-up

* cont : fix build

* cont : clarify padding token for gemma4

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Co-authored-by: Pranesh Gonegandla <pranesh.iitp@gmail.com>
Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>