CIMNotebook: Zazuko Interop & Format Conversion

[spah-soptim]

Objective: Ensure backwards compatibility with the Zazuko format and provide an easy migration path.

Description:
Existing .sparqlbook files (JSON-based) from Zazuko must open correctly and be convertible to our native markdown format.

• Register the cimnotebook-sparqlbook type (Priority: Option).
• Implement the SparqlBookSerializer.
• Provide the cimnotebook.notebook.convert command to translate between Markdown and Sparqlbook (writes a sibling file and opens it).

Acceptance Criteria:

• A Zazuko sample .sparqlbook opens successfully as a notebook.
• The conversion command successfully translates .sparqlbook to .cimnb.md and vice versa.
• Unit tests for the SparqlBookSerializer are implemented and passing.