llama.cpp permits attention.sliding_window_pattern as a scalar period as
well as a per-layer array; current writers for Gemma-family and other
architectures use the scalar form, which read_gguf_header/profile_from_gguf
silently dropped (isinstance(v, list) is False), sending unlisted
architectures back to the all-full-attention fallback this fix was meant to
avoid.
Add GGUFHeader.sliding_window_pattern_period and expand it per llama.cpp's
set_swa_pattern() semantics, including the small set of architectures
(cohere2moe, modern-bert, smallthinker, laguna) whose loader uses
dense_first=True instead of the default False.
Also add binary GGUF round-trip tests (real read_gguf_header() parsing, not
GGUFHeader built by hand): array pattern with non-bool element types, both
scalar dense_first variants, and a length-mismatched array falling back
safely.
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