fix: resolve chunks=-1 to chunk size 1 on zero-length axes - #4307

* fix: resolve chunks=False to chunk size 1 on zero-length axes

`chunks=False` means "one chunk covering the whole array", but on a
zero-length axis it built a chunk of size 0. For Zarr format 3 this was
rejected by RegularChunkGridMetadata with "integer chunk edge length
must be >= 1, got 0"; with shards="auto" it raised ZeroDivisionError on
both formats; and for Zarr format 2 it silently wrote `chunks: [0]`,
which corrupted reads after the axis was later resized.

Route `False` through the `-1` sentinel so the zero-length clamp added
in #4307 (max(span, 1), matching _guess_regular_chunks) lives in one
place. Both spellings now yield chunk size 1 on empty axes.

Rebased over #4218, which rewrote normalize_chunks_nd to build
FixedDimension/VaryingDimension grids: the False branch now falls
through to the -1 path instead of returning a ChunkGrid directly, and
the test table expectations use the new bare-int / tuple form.

Assisted-by: ClaudeCode:claude-fable-5-1

* chore: rename changelog fragment to the PR number

Assisted-by: ClaudeCode:claude-fable-5-1

`chunks=False` means "one chunk covering the whole array", but on a
zero-length axis it built a chunk of size 0. For Zarr format 3 this was
rejected by RegularChunkGridMetadata with "integer chunk edge length
must be >= 1, got 0"; with shards="auto" it raised ZeroDivisionError on
both formats; and for Zarr format 2 it silently wrote `chunks: [0]`,
which corrupted reads after the axis was later resized.

Route `False` through the `-1` sentinel so the zero-length clamp added
in zarr-developers#4307 (max(span, 1), matching _guess_regular_chunks) lives in one
place. Both spellings now yield chunk size 1 on empty axes.

Rebased over zarr-developers#4218, which rewrote normalize_chunks_nd to build
FixedDimension/VaryingDimension grids: the False branch now falls
through to the -1 path instead of returning a ChunkGrid directly, and
the test table expectations use the new bare-int / tuple form.

Assisted-by: ClaudeCode:claude-fable-5-1

…, clamps and metadata

Invariant: a chunk edge length is always >= 1; a dimension's extent may be 0,
in which case the dimension has zero chunks (ceildiv(0, size) == 0).

Zero-length-axis bugs have recurred since 2017 (#150, #241, #303, zarr-developers#972,
zarr-developers#1977, zarr-developers#2434, zarr-developers#3711, zarr-developers#4305, zarr-developers#4307, zarr-developers#4328) because the layers disagreed on
this invariant and every span-derived chunk spelling clamped on its own:

- The metadata layer (common.py, metadata/v3.py) required chunk edges >= 1,
  but the in-memory FixedDimension allowed size == 0 with four special-case
  branches left over from zarr-developers#2434, so normalization could build a grid the
  metadata constructor then rejected. FixedDimension now rejects size < 1
  and the four `if self.size == 0` branches are gone. VaryingDimension
  already required edges > 0 and is unchanged.
- `chunks=-1`, `chunks=False`, `chunks="auto"` (_guess_regular_chunks, both
  the typesize == 0 early return and the np.maximum line) and `shards="auto"`
  each derived "one chunk covering the axis" independently. They now all go
  through one helper, `_full_span_chunk_size(span) = max(span, 1)`, which is
  the single definition of that phrase for a possibly zero-length axis.
- Zarr format 2 metadata had no chunk >= 1 check, so a legacy `chunks: [0]`
  document opened fine and read uninitialised memory after a resize. It now
  raises a clear ValueError at parse time, matching the format 3 grid.
- Rectilinear grids had no creation-time spelling for a zero-length axis:
  normalize_chunks_1d required sum(edges) == span, which no list of positive
  edges can satisfy for span 0, even though the same state is reachable via
  resize((0,)) and round-trips through reopen. For span == 0 any non-empty
  list of positive edges is now accepted verbatim, producing the same
  VaryingDimension(edges, extent=0) that resize produces; the strict sum
  check is kept for span > 0.

Tests: the per-spelling regression test from zarr-developers#4328 is replaced by one matrix
over {-1, False, "auto", 1, (1,...), [[2, 2]]} x {(0,), (0, 4), (4, 0),
(0, 0), ()} x {v2, v3} x {no shards, shards="auto" with and without a byte
budget, explicit shards}, with separate small tests for each error case.
Tests that constructed FixedDimension(size=0) now assert it raises, and a
zero-extent test covers the behaviour the old special cases were guarding.

Assisted-by: ClaudeCode:claude-fable-5-1

…, clamps and metadata

Invariant: a chunk edge length is always >= 1; a dimension's extent may be 0,
in which case the dimension has zero chunks (ceildiv(0, size) == 0).

Zero-length-axis bugs have recurred since 2017 (#150, #241, #303, zarr-developers#972,
zarr-developers#1977, zarr-developers#2434, zarr-developers#3711, zarr-developers#4305, zarr-developers#4307, zarr-developers#4328) because the layers disagreed on
this invariant and every span-derived chunk spelling clamped on its own:

- The metadata layer (common.py, metadata/v3.py) required chunk edges >= 1,
  but the in-memory FixedDimension allowed size == 0 with four special-case
  branches left over from zarr-developers#2434, so normalization could build a grid the
  metadata constructor then rejected. FixedDimension now rejects size < 1
  and the four `if self.size == 0` branches are gone. VaryingDimension
  already required edges > 0 and is unchanged.
- `chunks=-1`, `chunks=False`, `chunks="auto"` (_guess_regular_chunks, both
  the typesize == 0 early return and the np.maximum line) and `shards="auto"`
  each derived "one chunk covering the axis" independently. They now all go
  through one helper, `_full_span_chunk_size(span) = max(span, 1)`, which is
  the single definition of that phrase for a possibly zero-length axis.
- Zarr format 2 metadata had no chunk >= 1 check, so a legacy `chunks: [0]`
  document opened fine and read uninitialised memory after a resize. It now
  raises a clear ValueError at parse time, matching the format 3 grid.
- Rectilinear grids had no creation-time spelling for a zero-length axis:
  normalize_chunks_1d required sum(edges) == span, which no list of positive
  edges can satisfy for span 0, even though the same state is reachable via
  resize((0,)) and round-trips through reopen. For span == 0 any non-empty
  list of positive edges is now accepted verbatim, producing the same
  VaryingDimension(edges, extent=0) that resize produces; the strict sum
  check is kept for span > 0.

Tests: the per-spelling regression test from zarr-developers#4328 is replaced by one matrix
over {-1, False, "auto", 1, (1,...), [[2, 2]]} x {(0,), (0, 4), (4, 0),
(0, 0), ()} x {v2, v3} x {no shards, shards="auto" with and without a byte
budget, explicit shards}, with separate small tests for each error case.
Tests that constructed FixedDimension(size=0) now assert it raises, and a
zero-extent test covers the behaviour the old special cases were guarding.

Assisted-by: ClaudeCode:claude-fable-5-1