Lazy-import visualization and xarray to avoid eager heavy dependencies

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Motivation

• Prevent the heavy visualization stack and xarray from being imported at package import time to reduce import-time failures and overhead.
• Provide backward-compatible verb/utility functions that only load visualization code when actually invoked.

Description

• Stop importing xarray eagerly in cubedynamics.__init__ by guarding the import with typing.TYPE_CHECKING and remove top-level viz imports; add lazy wrappers show_cube_lexcube and plot_median_over_space that import the real viz functions on call.
• In cubedynamics.verbs, remove top-level cubedynamics.viz and xarray imports and introduce a helper _import_xarray() for lazy xarray loading, and import cubedynamics.viz inside the show_cube_lexcube verb implementation.
• Adjust _unwrap_dataarray signature to avoid referencing xr at import time and use _import_xarray() where runtime access to xarray is required.
• Add a unit test tests/test_verbs_import_no_eager_viz.py that verifies importing cubedynamics.verbs does not eagerly load cubedynamics.viz or cubedynamics.viz.lexcube_viz.

Testing

• Added and ran the unit test tests/test_verbs_import_no_eager_viz.py, which asserts that importing cubedynamics.verbs does not place cubedynamics.viz or cubedynamics.viz.lexcube_viz into sys.modules, and the test passed.

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Codex Task