Merge perf/cache-default-chunk-spec into perf/prepared-write-v2

# Conflicts:
#	src/zarr/core/array.py

Author: d-v-b

changes/3908.misc.md

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

src/zarr/core/array.py

@@ -5402,6 +5402,14 @@ def _get_default_chunk_spec(
     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(