Closes #5278: alignment tests for arkouda.numpy.segarray

[ajpotts]

SegArray NumPy Alignment Test Suite

Summary

This PR adds and refines a NumPy alignment test suite for SegArray, exercising behavior against
well-defined NumPy-style reference semantics. The goal is to clearly distinguish:

• Correct behavior
• Known divergences / bugs (explicitly marked with xfail)
• Future work needed for full alignment

This improves test coverage while avoiding false negatives.

---

What’s Included

1. SegArray alignment tests

New/expanded tests in:

tests/numpy/alignment_verification/segarray_alignment.py

cover:

• Concatenation (concat)
• Uniqueness (unique)
• Set operations (intersect, union, setdiff, setxor)
• Aggregations (sum, etc.)

Each test compares Arkouda results against a NumPy-style reference implementation.

---

Known Issues Marked as xfail

The following behaviors are currently known issues and are explicitly marked as expected failures:

A. String concatenation

• SegArray.concat(axis=1) fails for string dtype
• Root cause: lack of multidimensional Strings support

B. unique() with empty segments

• SegArray.unique() fails or errors when empty segments are present
• Caused by invalid segment broadcasting

C. Set-ops constructing invalid segments

• Some set-ops construct segment labels instead of offsets
• Leads to ValueError: Segments must be unique and in sorted order

D. Set-ops ordering mismatch vs NumPy

• Arkouda preserves stable/input order
• NumPy returns sorted unique values
• Membership matches; ordering differs

E. Aggregations drop empty segments

• SegArray.sum() (and similar) drop empty segments
• Expected: one result per segment (e.g. sum([]) == 0)
• Actual: empty segments omitted

All of the above are documented inline with pytest.xfail(...) and explanatory messages.

---

Why This PR Is Useful

• Makes NumPy alignment expectations explicit
• Prevents regressions by locking in current behavior
• Separates correctness bugs from API-contract decisions
• Enables incremental fixes without rewriting tests

---

Follow-up Work (Out of Scope)

• Decide and document ordering semantics for set-ops
• Fix aggregation behavior for empty segments
• Implement multidimensional Strings
• Normalize segment construction in set-ops

---

Notes for Reviewers

• All xfails are narrow and intentional
• Removing an xfail should be paired with a corresponding fix
• No production behavior is changed in this PR

Closes #5278: alignment tests for arkouda.numpy.segarray

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