perf(indexing): speed up sorted 1-D coordinate selections

[selmanozleyen]

Summary

Speeds up CoordinateIndexer for sorted, in-bounds, one-dimensional integer selections on regular chunk grids.

Because the coordinates are already grouped by chunk, this uses searchsorted over the touched chunk boundaries instead of repeatedly processing the full selection.

Runs	Elements	main	This PR
1,024	1.48M	8.7 ms	0.5 ms
4,096	5.94M	35 ms	2.1 ms
8,192	11.9M	71 ms	4.5 ms

Closes #4170

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 — not applicable; no public API changes
• Update the user guide — not applicable; behavior is unchanged
• Changes documented 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 93.85%. Comparing base (80e00ae) to head (4b43d92).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4172      +/-   ##
==========================================
+ Coverage   93.84%   93.85%   +0.01%     
==========================================
  Files          91       91              
  Lines       12549    12573      +24     
==========================================
+ Hits        11777    11801      +24     
  Misses        772      772              

Files with missing lines	Coverage Δ
src/zarr/core/indexing.py	96.40% <100.00%> (+0.10%)	⬆️

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