Publish the marimo experiment notebooks in the book

[tschm]

Follow-up to #539, which removed five mkdocs.yml nav entries pointing at
notebooks/Experiment1.html–Experiment5.html because those pages are never built.
This issue is about actually publishing them.

Why it isn't a one-liner

rhiza-task exports notebooks from marimo_folder, which this repo leaves unset, so it
resolves to the CLI default docs/notebooks — a folder that doesn't exist here. Hence:

[WARN] no marimo folder; skipping notebook export

Setting marimo-folder = "book/marimo/notebooks" fixes the export, but breaks the marimo
workflow:

• The exporter globs folder.glob("*.py") and rhiza_marimo.yml's matrix globs
  find -maxdepth 1 -name "*.py". Neither filters for actual marimo notebooks.
• That folder also holds optimize.py and preamble.py, which are plain modules with no
  PEP 723 header. The matrix runs uv run --script <file> per match, so it would go from
  0 jobs (currently skipped, empty matrix) to 7, with those 2 failing.

What a real fix has to touch

book/marimo/notebooks currently doubles as source_folder (the importable modules) and
the notebook folder. Splitting them means:

• from preamble import … in all five Experiment*.py plus optimize.py — they reach it
  via sys.path.insert(0, str(Path(__file__).parent)), so preamble.py must stay a sibling
• the [tool.ty.environment] extra-paths PEP 723 headers in the notebooks
• the 1:1 mapping table in docs/development/TEST_LAYOUT.md
• scripts/check_test_layout.py
• source_folder, and the 100% coverage gate that points at it

Alternative

Leave the notebooks unpublished and keep them out of the nav (the state after #539). The
experiments are still exercised by tests/test_experiment1.py–5 and
tests/test_notebook_sharpe.py, so nothing is unverified — they're just not on the site.