feat(Mf6Splitter): split structured models into DISV models

[jdhughes-dev]

• split_model() and split_multi_model() take to_disv, which writes DISV models from a
  structured model instead of the structured bounding box of each part
• a DISV model carries only the cells assigned to it, and stacks of cells that are inactive in
  every layer are excluded
• to_disv is ignored with a warning for a DISV model and is an error for a DISU model
• adds StructuredGrid.get_cell_iverts(); each model grid is built from it one model at a time,
  so peak memory scales with the largest part rather than with the parent grid
• save_node_mapping() records the grid type of the split models so a structured to DISV mapping
  round trips

Closes #2816

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 73.4%. Comparing base (556c088) to head (00e5aae).
⚠️ Report is 209 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2820      +/-   ##
===========================================
+ Coverage     55.5%    73.4%   +17.9%     
===========================================
  Files          644      659      +15     
  Lines       124135   132344    +8209     
===========================================
+ Hits         68947    97239   +28292     
+ Misses       55188    35105   -20083     

Files with missing lines	Coverage Δ
flopy/discretization/structuredgrid.py	55.5% <100.0%> (+8.1%)	⬆️
flopy/mf6/utils/model_splitter.py	66.9% <100.0%> (-8.0%)	⬇️

... and 583 files with indirect coverage changes

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[Copilot]

Pull request overview

This PR enhances Mf6Splitter to optionally convert structured (DIS) models into DISV (vertex) models during splitting, so each split model contains only its assigned active cells (excluding cell stacks inactive in every layer) while preserving the ability to reconstruct outputs and round-trip saved node mappings.

Changes:

• Add to_disv option to split_model() and split_multi_model() to write DISV models when splitting a structured model, with validation/warnings for incompatible grid types.
• Add StructuredGrid.get_cell_iverts() and use it to build DISV vertices/cell2d per split model with reduced peak memory usage.
• Extend node-mapping save/load to record new_grid_type, and add new automated tests for structured→DISV splitting, idomain handling, geometry, and reconstruction.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
flopy/mf6/utils/model_splitter.py	Implements to_disv workflow, DIS→DISV remapping, grid-type persistence in node maps, and related reconstruction adjustments.
flopy/discretization/structuredgrid.py	Adds StructuredGrid.get_cell_iverts() and reuses it for structured iverts generation.
autotest/test_model_splitter.py	Adds coverage for structured→DISV splitting behavior, idomain exclusion rules, reconstruction, geometry, and save/load mapping.
autotest/test_grid.py	Adds unit test coverage for StructuredGrid.get_cell_iverts().

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[jdhughes-dev]

Fixed in e98301e. nodes is now np.ravel(np.asarray(nodes, dtype=int)), which handles an array of any shape and coerces floats. The ravel returns a view and asarray() is a no-op for an int array, so the node array is not copied on the path that matters, where get_cell_iverts() is called once per split model. The (n, 1) and ndarray cases were added to test_get_cell_iverts.