Function write_mphf_container 

pub fn write_mphf_container<W: Write + Seek>(
    writer: &mut W,
    mphfs: &[Option<&PartitionedMphf>],
) -> Result<Vec<MphfPartitionEntry>>

Helper functions for MPHF container operations

Write MPHFs to a container format

Creates a container file with:

• Header with num_partitions
• Offset table (partition_id, byte_offset, byte_size) for each partition
• Serialized MPHF data concatenated

Returns the offset table for reference