create_checkpoint uses non-existent .data field instead of .state_data

[elias-ba]

In lib/lightning/collaboration/persistence_writer.ex, the create_checkpoint/1 function references .data on lines 457 and 462:

Yex.apply_update(temp_doc, latest_checkpoint.data)
Yex.apply_update(temp_doc, update_record.data)

However, the DocumentState schema only has state_data:

field :state_data, :binary

This will cause a KeyError crash when a document reaches 500 updates and triggers checkpoint creation. The PersistenceWriter GenServer will crash, potentially losing pending updates.