perf: cache default ArraySpec for regular chunk grids

[d-v-b]

For regular grids, all chunks have the same codec_shape, so we can
build the ArraySpec once and reuse it for every chunk — avoiding the
per-chunk ChunkGrid.getitem + ArraySpec construction overhead.

Adds _get_default_chunk_spec() and uses it in _get_selection and
_set_selection. Saves ~5ms per 1000 chunks.

Co-Authored-By: Claude Opus 4.6 (1M context) noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.24%. Comparing base (80d45ca) to head (ff08891).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3908   +/-   ##
=======================================
  Coverage   93.23%   93.24%           
=======================================
  Files          87       87           
  Lines       11696    11702    +6     
=======================================
+ Hits        10905    10911    +6     
  Misses        791      791           

Files with missing lines	Coverage Δ
src/zarr/core/array.py	97.72% <100.00%> (+0.01%)	⬆️

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[codspeed-hq]

Merging this PR will improve performance by 11.1%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 4 improved benchmarks
✅ 62 untouched benchmarks
⏩ 6 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	test_slice_indexing[None-(slice(None, None, None), slice(None, None, None), slice(None, None, None))-memory]	389.8 ms	352.5 ms	+10.59%
⚡	WallTime	test_slice_indexing[None-(slice(0, None, 4), slice(0, None, 4), slice(0, None, 4))-memory]	386.7 ms	348.3 ms	+11.01%
⚡	WallTime	test_slice_indexing[None-(slice(10, -10, 4), slice(10, -10, 4), slice(10, -10, 4))-memory]	211.6 ms	190.4 ms	+11.1%
⚡	WallTime	test_slice_indexing[None-(slice(None, None, None), slice(0, 3, 2), slice(0, 10, None))-memory_get_latency]	4.3 ms	3.9 ms	+10.54%

---

_{Comparing d-v-b:perf/cache-default-chunk-spec (6fa93cb) with main (029c376)²}

Footnotes

1. 6 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (c4730be) during the generation of this report, so 029c376 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩