Function encode_changeset 

pub fn encode_changeset(
    page_size: u32,
    pages: &mut [PageEntry],
) -> Result<Vec<u8>>

Encode pages into a deterministic changeset byte stream.

Canonicalization rule:

• sort pages by (page_number, page_xxh3, page_bytes) before encoding.
• this removes non-deterministic map-iteration effects from changeset_bytes.

The encoded stream is self-delimiting via the total_len field in the header.

Errors

Returns error if page_size is 0 or pages are empty.