Build datasets in notebooks without ipywidgets; fix datasets overview example

[solarsys]

Problem

When a dataset builds its event cache in a Jupyter kernel, BaseDataset calls dask's progress(). In a kernel, dask draws an ipywidgets progress bar, but ipywidgets is not a PyHealth dependency. In a notebook without it, such as a plain venv kernel in VS Code or Cursor, building any dataset fails:

ModuleNotFoundError: No module named 'ipywidgets'
Error during caching, removing incomplete file …/global_event_df.parquet

The same unguarded call is in the 2.0.2 release.

Changes

• base_dataset.py:
  • When ipywidgets isn't importable, pass notebook=False to dask's progress(), so dask falls back to its text bar. With ipywidgets installed, behaviour is unchanged.
  • In notebooks, where dask uses threaded workers, wait 0.3 s after the compute before closing the cluster. Without this, closing the cluster races dask's 100 ms progress feed and logs a spurious CancelledError traceback, even though the data was written correctly. It's a pragmatic workaround and only runs in notebooks; happy to replace it if there's a cleaner hook.
• tests/core/test_base_dataset.py: simulates a kernel without ipywidgets, as in Jupyter (threaded workers, kernel detection on). It asserts that the build succeeds and that the dask scheduler logs no errors. The test fails on master with the ModuleNotFoundError above and passes with this change.
• examples/datasets_overview.ipynb: the synthetic-samples cell called SampleDataset(samples=…). That constructor no longer exists; the committed output already showed an error, and running it today raises TypeError: … missing … 'path'. It now uses create_sample_dataset(...), and its stale error output is cleared. The new cell runs and prints Loaded 2 synthetic samples. No other cell changed.
• docs/install.rst: a note on notebooks and the optional ipywidgets, under Platform-Specific Notes.

Verification

• tools/check_pr_rules.py: all rules pass.
• Full core suite: 1,382 tests. Everything passes except the 18 tests in test_medcode_icd_mappings (medcode: ICD-9 <-> ICD-10 translation and grouper vocabularies via icd-mappings #1257), which fail locally only because icd-mappings wasn't installed in my older environment. With it installed, those tests and test_base_dataset pass (47/47).

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