Function serialize_pages 

pub fn serialize_pages<I>(
    blobs: &dyn BlobStore,
    meta: CacheMeta,
    pages: I,
    logical_seqs: &[LogicalSeq],
) -> Result<Digest256>
where
    I: IntoIterator<Item = (u32, PageBytes)>,

Serialize a list of (ix, K-bytes, V-bytes) page tuples + the per-sequence logical mapping into a PageManifest and persist every blob through blobs. Returns the digest of the manifest itself, ready to drop into the .pfimg cache.manifest field.