Keep cached measurements when only the pointer shape changes

[trissim]

Changing the pointer style was treated like any unknown enum change, so it dropped cached content measurements and subtree geometry although it only changes the terminal pointer. StringEnumProperty in _style_properties.py now takes a geometry argument that defaults to true, and Styles.pointer in styles.py sets it to false. Other enum styles such as text wrapping, position and constrain keep their geometry effect. Tests are added to test_programmatic_style_changes.py.

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[trissim]

Reviewed descriptor scope: pointer alone declares geometry=False; enum geometry remains conservative independently of scheduling layout. Native Content resources retain measurements, custom pointer-reading rendering remains sensitive, wrapping still invalidates. Source change is ready; current Parent channel-header delivery remains on its explicitly checked c330 cohort and does not claim this cursor fix is installed.