CIMNotebook: Base Infrastructure & Markdown Round-Trip

[spah-soptim]

Objective: Establish the VS Code frontend foundation. *.cimnb.md files open as notebooks and save losslessly.

Description:
The VS Code extension needs native notebook support using a markdown-first approach for git-friendly diffs. Initially, this focuses on authoring (no execution).

• Register the notebook types (cimnotebook for *.cimnb.md and cimnotebook-markdown as an option for regular *.md).
• Implement the pure types (cells.ts).
• Implement the MarkdownNotebookSerializer (parsing & serializing).
• Crucial: The parser must be strictly idempotent (round-trip stable: serialize(parse(x)) == x) so read-only operations don't create dirty git states. Unknown code fences must remain untouched as markdown.

Acceptance Criteria:

• *.cimnb.md opens as a notebook by default.
• Regular .md files can be opened via "Open With... -> CIM Notebook".
• Pure TS unit tests (node:test) for the markdown serializer pass.
• docs/content/cimnotebook/notebooks.md is created and registered in sidebars.js.
• CI/CD (Linter, tsc) passes without errors.