[codex] Document marimo integration notes

[jlewi]

Summary

Adds a design note capturing marimo architecture details relevant to possible Runme integration.

The note covers:

• frontend/backend execution flow and document transactions
• how marimo uses its in-memory kernel graph
• native .py notebook serialization and cell delineation
• file/path usage and why execution does not fundamentally require the kernel to read the notebook file
• implications for moving notebook storage into Runme or the frontend
• references to relevant marimo GitHub issues and discussions

Validation

Docs-only change. No build or test suite was run.