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 93.11%. Comparing base (029c376) to head (6fa93cb).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3908   +/-   ##
=======================================
  Coverage   93.11%   93.11%           
=======================================
  Files          85       85           
  Lines       11365    11371    +6     
=======================================
+ Hits        10582    10588    +6     
  Misses        783      783           

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[maxrjones]

given the name of _get_default_chunk_spec, should this be a ChunkSpec?

[d-v-b]

it's the arrayspec for a chunk, not a chunkspec. And the consumer needs an ArraySpec, so we can't change the return type.

[maxrjones]

ok, still a bit confusing but I guess that might relate to the fact that a better design wouldn't need a per-chunk ArraySpec.

My only other question is why build a new function rather than making chunk_coords: tuple[int, ...] optional in _get_chunk_spec, such that it could return a default?

[d-v-b]

what would the default chunk coordinates be? the "origin" chunk coordinate depends on the dimensionality of the array

[d-v-b]

Part of the goal here is to avoid object creation overhead inside _get_chunk_spec. Adding default parameters to _get_chunk_spec would not help us, because we would still create many identical ArraySpec objects. The only change to _get_chunk_spec that would help is adding a caching layer via @lru_cache, I should see if that works