Update notebooks to use new xr.Dataset API

[drbenvincent]

Depends on: #849 must be merged first.

Update notebooks to use new xr.Dataset API

Following the consolidation of experiment design-matrix attributes into xr.Dataset objects (#199), the old attribute names (pre_X, pre_y, post_X, post_y, datapre_control, datapre_treated, datapost_control, datapost_treated, X, y) are deprecated. They still work via backward-compatible @property accessors that emit DeprecationWarning, but notebooks should be migrated to the new API.

New API

Experiment type	Old attributes	New access pattern
InterruptedTimeSeries	result.pre_X, result.pre_y, result.post_X, result.post_y	result.pre_design["X"], result.pre_design["y"], result.post_design["X"], result.post_design["y"]
SyntheticControl	result.datapre_control, result.datapre_treated, result.datapost_control, result.datapost_treated	result.pre_design["control"], result.pre_design["treated"], result.post_design["control"], result.post_design["treated"]
Formula-based (DiD, RD, RK, PrePostNEGD, PiecewiseITS, PanelRegression)	result.X, result.y	result.design["X"], result.design["y"]

Affected notebooks

These notebooks access deprecated attributes directly and should be updated:

• docs/source/notebooks/did_pymc.ipynb — result.causal_impact (not deprecated, but review for other attrs)
• docs/source/notebooks/did_pymc_banks.ipynb — result.causal_impact
• docs/source/notebooks/rd_donut_pymc.ipynb — result.discontinuity_at_threshold, result.fit_data
• docs/source/notebooks/its_pymc.ipynb — result.post_impact, result.datapost
• docs/source/notebooks/sc_pymc.ipynb — result.post_impact_cumulative
• docs/source/notebooks/geolift1.ipynb — result.post_impact_cumulative

Notes

• The deprecated properties will be removed in a future release.
• Notebooks should be re-run after updating to verify outputs are unchanged.
• Some notebooks may access attributes that were not deprecated (e.g., result.causal_impact, result.post_impact); these should be reviewed but may not need changes.