Function derive_kv_ns 

pub fn derive_kv_ns(
    model_fp: u64,
    layout: &GroupLayout,
    elem_bytes: u32,
    block_size: u32,
    head_dim: u32,
    client_salt: u64,
) -> NonZeroU64

Derive the KV paging namespace. FROZEN tagged encoding (injective: fixed-width [tag][8-byte LE] records) — any field or order change is a deliberate fleet cache flush and must bump KV_NS_VERSION:

tag	field	tag	field	tag	field
0x00	KV_NS_VERSION	0x04	block_size	0x08	num_kv_heads
0x01	model_fp	0x05	head_dim	0x09	fs_block_size
0x02	KV_DOMAIN	0x06	num_layers	0x0a	group_stride
0x03	elem_bytes	0x07	num_blocks	0x0b	client_salt

model_fp carries the quant identity + model_type + ATLAS_MODEL_ID salt (ModelFingerprint::derive_kv in spark-model); the geometry fields make the layout identity explicit because group_id numbering is layout-relative. Zero-avoidance falls back to FNV_OFFSET (p = 2^-64), keeping the result total — ns = 0 stays unrepresentable end-to-end.