fix: split a docstring that follows code into its own notebook cell

[Jammy2211]

Closes the generator half of #211.

What was broken

ipynb-py-convert's py2nb splits cells on the literal "\n\n# %%\n" — the marker must be preceded by a blank line. The docstring-opener branch of add_notebook_quotes emitted "# %%\n" after a single newline, so a column-0 docstring opened on the line immediately after code never split. The marker and both delimiters ended up inside the preceding code cell as literal text:

np.save(
    file=dataset_path / f"nufft_precision_operator_{mask_radius}.npy",
    arr=nufft_precision_operator,
    allow_pickle=False,
)
# %%
'''
To load the `nufft_precision_operator` matrix from hard-disk in your model-fit, you can use the code:
'''

That cell is a SyntaxError if a user runs it. 13 committed notebook code cells were broken this way, across autolens_workspace (4), autogalaxy_workspace (3), autocti_workspace (4) and HowToLens (2).

The fix

Emit the separator when it is missing, under two constraints — both load-bearing, both verified:

1. Never when out is empty. py2nb strips a leading # %%\n header; a leading blank line defeats that strip and yields a spurious empty first code cell.
2. Never when the output already ends blank. Emitting it unconditionally appends a trailing blank line to every code cell in every generated notebook. Verified empirically: regenerating autofit_workspace with this change produces a zero-byte diff, and HowToLens changes in exactly its 2 broken notebooks and nothing else.

Also raises on a hand-written column-0 # %% in a source script. This function is what inserts cell markers, so an authored one collides with the generated one and corrupts the conversion the same way — guides/hpc/example_cpu_and_gpu.py in two workspaces carried these for years. Loud beats laundering it into a broken artefact.

Tests

test	pins
test_docstring_immediately_after_code_is_its_own_markdown_cell	the bug. Fails on main: ['markdown', 'code', 'code'] instead of ['markdown', 'code', 'markdown', 'code']
test_hand_written_cell_marker_in_source_raises	the new guard
test_script_ending_in_code_keeps_a_complete_final_code_cell	that a script ending in code converts intact — the pin that makes the workspaces' trailing """Finish.""" crutch provably unnecessary
test_leading_docstring_does_not_produce_an_empty_first_code_cell	constraint 1

The two pre-existing adjacent-docstring tests pass unchanged.

6 passed in 0.43s

Merge order

This must merge before the workspace PRs in #211 — workspaces invoke ../PyAutoHands/autohands/generate.py from the local checkout, so their regenerated notebooks are only correct against this version.

🤖 Generated with Claude Code

https://claude.ai/code/session_013XzXdQMU3KV2tyZaMNPvsM