perf: cache default ArraySpec for regular chunk grids (#3908)

* perf: cache default ArraySpec for regular chunk grids

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>

* docs: changelog

* refactor: inline creation of default arrayspec

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: d-v-b

changes/3908.misc.md

@@ -0,0 +1 @@
+Reuse a constant `ArraySpec` during indexing when possible.

src/zarr/core/array.py

@@ -5427,11 +5427,24 @@ async def _get_selection(
 
         # reading chunks and decoding them
         indexed_chunks = list(indexer)
+        # For regular grids, all chunks share the same ArraySpec, so build it once
+        # and reuse it to avoid per-chunk ChunkGrid lookups and ArraySpec construction.
+        regular_grid = chunk_grid.is_regular
+        if regular_grid:
+            regular_chunk_spec = ArraySpec(
+                shape=chunk_grid.chunk_shape,
+                dtype=metadata.dtype,
+                fill_value=metadata.fill_value,
+                config=_config,
+                prototype=prototype,
+            )
         results = await codec_pipeline.read(
             [
                 (
                     store_path / metadata.encode_chunk_key(chunk_coords),
-                    _get_chunk_spec(metadata, chunk_grid, chunk_coords, _config, prototype),
+                    regular_chunk_spec
+                    if regular_grid
+                    else _get_chunk_spec(metadata, chunk_grid, chunk_coords, _config, prototype),
                     chunk_selection,
                     out_selection,
                     is_complete_chunk,
@@ -5770,11 +5783,24 @@ async def _set_selection(
         _config = replace(_config, order=order)
 
     # merging with existing data and encoding chunks
+    # For regular grids, all chunks share the same ArraySpec, so build it once
+    # and reuse it to avoid per-chunk ChunkGrid lookups and ArraySpec construction.
+    regular_grid = chunk_grid.is_regular
+    if regular_grid:
+        regular_chunk_spec = ArraySpec(
+            shape=chunk_grid.chunk_shape,
+            dtype=metadata.dtype,
+            fill_value=metadata.fill_value,
+            config=_config,
+            prototype=prototype,
+        )
     await codec_pipeline.write(
         [
             (
                 store_path / metadata.encode_chunk_key(chunk_coords),
-                _get_chunk_spec(metadata, chunk_grid, chunk_coords, _config, prototype),
+                regular_chunk_spec
+                if regular_grid
+                else _get_chunk_spec(metadata, chunk_grid, chunk_coords, _config, prototype),
                 chunk_selection,
                 out_selection,
                 is_complete_chunk,