fix(datapipes): write domain-level global_data in SetGlobalField

[peterdsharpe]

What

SetGlobalField.apply_to_domain broadcast the injected fields to every sub-mesh's global_data, but never wrote them to the DomainMesh's own domain-level global_data. Any code reading DomainMesh.global_data never saw the injected fields.

• The domain path now writes both records.
• Sub-mesh behavior is unchanged.
• Plain Mesh behavior is unchanged.

Tests

New test/datapipes/transforms/test_set_global_field.py:

• domain-level global_data receives the field
• the sub-mesh broadcast still happens
• existing domain-level fields are preserved
• injecting an existing key overwrites it, matching the documented contract
• plain Mesh path unchanged

Full transforms suite passes; ruff check/format clean.

Notes

• Found while integrating a DomainMesh-native model into the unified external-aero recipe, which currently carries a local workaround (SetDomainGlobalField) that this fix makes unnecessary.
• A companion transform (ComputeUnitGlobalVector) that was previously bundled in this PR is now split out into draft PR feat(datapipes): add ComputeUnitGlobalVector transform #1825, since it is new API rather than a bug fix.

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[greptile-apps]

Greptile Summary

This PR fixes SetGlobalField for DomainMesh inputs. The main changes are:

• Domain-level global_data now receives injected fields.
• Existing per-sub-mesh global_data broadcast is preserved.
• Tests cover domain insertion, preservation, overwrite behavior, and plain Mesh behavior.
• The changelog documents the fixed transform behavior.

Important Files Changed

Filename	Overview
physicsnemo/datapipes/transforms/mesh/transforms.py	Adds a SetGlobalField.apply_to_domain override that updates domain-level global_data after preserving the existing sub-mesh broadcast.
test/datapipes/transforms/test_set_global_field.py	Adds tests for the fixed DomainMesh path and confirms the plain Mesh path still works.
CHANGELOG.md	Documents that SetGlobalField now writes injected fields to DomainMesh.global_data.

_{Reviews (1): Last reviewed commit: "Merge branch 'main' into psharpe/globald..." | Re-trigger Greptile}