Function kv_miss_error 

pub fn kv_miss_error(layer: u32, block: u32) -> Error

The hard-error a KV paging GET miss maps to. StorageBackend::read has no miss channel and an evicted KV block is UNRECOVERABLE without recomputing the prefix (unlike SSM snapshots, which recompute on miss by design) — so ST_MISS is corruption-equivalent and must fail loudly, naming the peer config that makes the KV kind miss-proof.