polish(zarr-indexing): re-review minors — step-zero ValueError, kw-only Partition, strides docs

- slice step zero now raises ValueError, matching NumPy in the wrapper's
  positional dialect (was IndexError)
- Partition is keyword-only: box was inserted mid-field-list, so
  positional construction would silently misbind
- strides() documents the empty-box case (bounding_box None, strides
  still defined)

Assisted-by: ClaudeCode:claude-fable-5

Author: d-v-b

packages/zarr-indexing/src/zarr_indexing/boundary.py

@@ -93,7 +93,7 @@ def _normalize_slice(sel: slice, size: int, axis: int) -> tuple[int, int, int]:
         raise IndexError(f"slice step must be an integer; got {sel.step!r}")
     step = 1 if sel.step is None else int(sel.step)
     if step == 0:
-        raise IndexError(f"slice step cannot be zero (axis {axis})")
+        raise ValueError(f"slice step cannot be zero (axis {axis})")  # ValueError: NumPy parity
     start, stop, step = sel.indices(size)
     stop = max(stop, start) if step > 0 else min(stop, start)
     return start, stop, step

packages/zarr-indexing/src/zarr_indexing/lazy_array.py

@@ -538,7 +538,7 @@ def _covers_whole_part(local_transform: IndexTransform, part_shape: tuple[int, .
     return True
 
 
-@dataclass(frozen=True)
+@dataclass(frozen=True, kw_only=True)
 class Partition:
     """One box of a `LazyArray`'s partitioning, as it falls through the view.
 
@@ -838,7 +838,11 @@ def strides(self) -> tuple[int, ...] | None:
         tuple of int, or None
             One positive stride per storage dimension, or `None` when
             [`is_box`][zarr_indexing.lazy_array.LazyArray.is_box] is false: a
-            query's coordinates are a lookup table and have no step.
+            query's coordinates are a lookup table and have no step. An
+            **empty** box still reports its strides even though
+            [`bounding_box`][zarr_indexing.lazy_array.LazyArray.bounding_box]
+            returns `None` — the step is a property of the selection's shape,
+            not of the (empty) region it touches.
 
         Notes
         -----

packages/zarr-indexing/tests/test_lazy_array.py

@@ -1173,7 +1173,7 @@ def test_a_reversed_view_is_re_based_to_origin_zero() -> None:
 
 
 def test_zero_step_is_rejected() -> None:
-    with pytest.raises(IndexError, match="step cannot be zero"):
+    with pytest.raises(ValueError, match="step cannot be zero"):
         make_source("numpy-whole").lazy[::0]