copy _grounds in clone() and use other.is_ground(v) in tensor()

[dlyongemallo]

Description

Cloned graphs lost their ground vertices because GraphS.clone() and Multigraph.clone() were not copying _grounds, and tensor() was checking g.is_ground(v) (the destination) instead of other.is_ground(v) (the source) when copying. This change fixes both of these bugs.

Related issue

N/A (but found this bug while working on #430)

Checklist

• I have added/updated tests (for bugfixes, please include a regression test, for new features, include new tests either to an existing test file or a new file).
• For new features: I have updated the documentation.
• All the functions I added have a docstring parsable by sphinx (for a good example see pyzx.extract.extract_circuit).
• I have added an entry to CHANGELOG.md describing my change.

[Copilot]

Pull request overview

Fixes loss of “ground” vertices when duplicating or combining graphs, ensuring hybrid (grounded) structures survive clone() and tensor() operations.

Changes:

• Copy _grounds in GraphS.clone() and Multigraph.clone() so cloned graphs preserve ground status.
• Fix BaseGraph.tensor() to query other.is_ground(v) when copying vertices from the source graph.
• Add regression tests for clone/tensor ground preservation and document the fix in the changelog.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/test_graph.py	Adds regression tests for ground preservation in clone() and tensor().
pyzx/graph/multigraph.py	Ensures _grounds is copied in Multigraph.clone().
pyzx/graph/graph_s.py	Ensures _grounds is copied in GraphS.clone().
pyzx/graph/base.py	Fixes tensor() to preserve ground status from the source graph (other).
CHANGELOG.md	Records the bugfix for clone() and tensor() ground handling.

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