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>

Author: d-v-b

src/zarr/core/array.py

@@ -5778,6 +5778,37 @@ def _get_chunk_spec(
     )
 
 
+def _get_default_chunk_spec(
+    metadata: ArrayMetadata,
+    chunk_grid: ChunkGrid,
+    array_config: ArrayConfig,
+    prototype: BufferPrototype,
+) -> ArraySpec | None:
+    """Build an ArraySpec for the regular (non-edge) chunk shape, or None if not regular.
+
+    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.
+
+    .. note::
+        Ideally the per-chunk ArraySpec would not exist at all: dtype,
+        fill_value, config, and prototype are constant across chunks —
+        only the shape varies (and only for edge chunks). A cleaner
+        design would pass a single ArraySpec plus a per-chunk shape
+        override, which ChunkTransform.decode_chunk already supports
+        via its ``chunk_shape`` parameter.
+    """
+    if chunk_grid.is_regular:
+        return ArraySpec(
+            shape=chunk_grid.chunk_shape,
+            dtype=metadata.dtype,
+            fill_value=metadata.fill_value,
+            config=array_config,
+            prototype=prototype,
+        )
+    return None
+
+
 async def _get_selection(
     store_path: StorePath,
     metadata: ArrayMetadata,
@@ -5857,11 +5888,16 @@ async def _get_selection(
 
         # reading chunks and decoding them
         indexed_chunks = list(indexer)
+        # Pre-compute the default chunk spec for regular grids to avoid
+        # per-chunk ChunkGrid lookups and ArraySpec construction.
+        default_spec = _get_default_chunk_spec(metadata, chunk_grid, _config, prototype)
         results = await codec_pipeline.read(
             [
                 (
                     store_path / metadata.encode_chunk_key(chunk_coords),
-                    _get_chunk_spec(metadata, chunk_grid, chunk_coords, _config, prototype),
+                    default_spec
+                    if default_spec is not None
+                    else _get_chunk_spec(metadata, chunk_grid, chunk_coords, _config, prototype),
                     chunk_selection,
                     out_selection,
                     is_complete_chunk,
@@ -6200,11 +6236,14 @@ async def _set_selection(
         _config = replace(_config, order=order)
 
     # merging with existing data and encoding chunks
+    default_spec = _get_default_chunk_spec(metadata, chunk_grid, _config, prototype)
     await codec_pipeline.write(
         [
             (
                 store_path / metadata.encode_chunk_key(chunk_coords),
-                _get_chunk_spec(metadata, chunk_grid, chunk_coords, _config, prototype),
+                default_spec
+                if default_spec is not None
+                else _get_chunk_spec(metadata, chunk_grid, chunk_coords, _config, prototype),
                 chunk_selection,
                 out_selection,
                 is_complete_chunk,