bug: marimo WASM ddd_tour notebook broken — polars import crashes pyodide

[paddymul]

Summary

The marimo WASM notebook at /example_notebooks/buckaroo_ddd_tour/ is completely broken. The page loads but renders nothing.

Root cause

buckaroo_ddd_tour.py has a cell that calls ddd.pl_df_with_weird_types_as_pandas(), which internally does import polars as pl (ddd_library.py:179). Polars has no pyodide/WASM wheel, so this cell crashes at runtime in the browser.

The failure cascades through marimo's reactive dependency graph:

1. pl_df_with_weird_types cell errors → pl_df_with_weird_types_config is never defined
2. The dropdown aggregator cell lists pl_df_with_weird_types_config as a required parameter → can't run → dropdown_dict is never defined
3. The main display cell depends on dropdown_dict → can't render

One broken cell poisons the entire notebook — even the cells that don't touch polars.

Introduced in

PR #641 added pl_df_with_weird_types / pl_df_with_weird_types_as_pandas to ddd_library.py and wired them into buckaroo_ddd_tour.py.

Why tests didn't catch it

• WASM Playwright tests only cover buckaroo_simple.py, not buckaroo_ddd_tour.py (full WASM suite deferred per Re-enable full WASM marimo Playwright test suite #513)
• Python unit tests run in CPython where polars is installed — ddd_library.py works fine natively
• RTD build generates the HTML but never runs it in a browser
• The polars import is inside a function body, so it passes static analysis and all native tests

Possible fixes

• Exclude the pl_df_with_weird_types cell from the WASM notebook (simplest)
• Guard with try/except ImportError so the cell gracefully degrades instead of crashing
• Make the dropdown aggregator not depend on pl_df_with_weird_types_config (break the cascade)