fix: unsigned index arrays raise IndexError because np.diff wraps and uint64 promotes

[selmanozleyen]

Hi,

fixes: #4285

Summary

An integer array selection with an unsigned dtype could fail where the same values in a
signed dtype worked. is_integer_array() accepts dtype.kind in "ui", so these selections
pass validation and are then mishandled — the dtype was never normalized to intp, which
IntArrayDimIndexer already claims in its annotation.

• Order.check used np.diff, which wraps on unsigned dtypes: np.diff(uint8([3, 0])) is
  253, not -3. A descending selection was classified INCREASING, so the argsort
  grouping indices by chunk was skipped and the lookup hit the wrong chunk.
• uint64 promotes to float64 against the signed chunk offset, on [] and .vindex
  alike, sorted or not.

Both raised rather than returning wrong data. Fixed by comparing instead of subtracting in
Order.check, and casting to intp in IntArrayDimIndexer and CoordinateIndexer.

For reviewers

The CoordinateIndexer cast sits after is_coordinate_selection on purpose — above it, a
float coordinate array would be silently truncated instead of raising. No test pins that.

Selections that previously raised now succeed; nothing that previously worked changes.

Author attestation

• I am a human, these are my changes, and I have reviewed and understood every change and can explain why each is correct.

TODO

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/user-guide/*.md
• Changes documented as a new file in changes/
• GitHub Actions have all passed
• Test coverage is 100% (Codecov passes)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.19%. Comparing base (d34f58a) to head (1b6010a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4286   +/-   ##
=======================================
  Coverage   94.19%   94.19%           
=======================================
  Files          92       92           
  Lines       12825    12826    +1     
=======================================
+ Hits        12080    12081    +1     
  Misses        745      745           

Files with missing lines	Coverage Δ
src/zarr/core/indexing.py	96.42% <100.00%> (+<0.01%)	⬆️

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